Introduction Occupational exposure to benzene has been demonstrated to increase the frequency of peripheral blood lymphocytes exhibiting chromosomal damage in a number of studies (reviewed in Dean@ 1985). In vivo exposure to benzene has been shown also to Induce chromosomal aberrations (CAs)@ micronuclei (MN)@ and sister chromatid exchanges (SCEs) in bone marrow cells of a number of animal species (e@ g .@ hamsters@ mouse@ rats) by a variety of exposure routes (inhalation@ oral gavage@ subcutaneous@ intraperitoneal) and concentrations (Dean@ 1985). The majority of these studies involved acute exposures to benzene atrelatively high doses by aroute (s) generally not pertinent to human occupational exposure situations. Only three bone marrow cytogenetic studies (Kissling and Speck@ 1971; Cortina et al.@ 1982; NTP@ 1985) have Involved exposure durations longer than two weeks@ and@ of these three studies@ only one (Cortina et al.@ 1982) involved exposure of animals to benzene by Inhalation@ the route most pertinent to occupational settings. From the acute exposure studies@ several conclusions on the bone marrow genotoxicity of benzene@ as measured by the induction of CAs@ MN and/or SCEs@ can be made. First@ male animals (mice and rats) exhibit a greater level of genotoxic damage than female animals (Dean@ 1969; Meyne and Legator@ 1980; Tice et al.@ 1980; Siou et al.@ 1981; Tice et a l.@ 1982; Gad-El-Karim et al.@ 1984; NTP@ 1985). Second@ the magnitude of the response is strain and age-dependent (Tice et al.@ 1982; Luke et al.@ 1985). Third@ the induction of damage is dependent upon the metabolism of benzene to reactive species (Tice et al.@ 1982; Gad-El-Karim et al.@ 1984). Fourth@ acute exposures to relatively low doses of benzene (1-30 ppm) for short durations (4 hr to 2 dy) can induce a significant increase in SCEs and MN in animals (Tice et al. @ 1982; Toft et al.@ 1982; Tice et al.@ 1984; Erexson et al.@ 1983). . Finally@ mice appear to be more sensitive than rats (Cortina et al.@ 1982). What has not been examined experimentally insufficient detail is the time-dependent effect of chronic exposure conditions (i.e.@ gt;l month) on the induction of bone narrow genotoxic damage. Typically@ the experiments involving chronic exposure conditions have assessed bone marrow damage at the termination of the exposure duration only (e.g.@ Cortina et a l.@ 1982) and have not evaluated whether the magnitude of the genotoxic response varied during the exposure period. Recently@ MacGregor and coworkers (MacGregor et al.@ 1980; Schlegal and MacGregor@ 1982) have introduced a variation of the erythrocyte micronucleus test developed by Schmid (1976) which permits a time-dependent evaluation of bone marrow genotoxic damage in mice induced during chronic exposures. Micronuclei are cytoplasmic nuclear bodies resulting from the exclusion of acentric chromosomal fragments or lagging chromosomes from daughter nuclei during cytokinesis (Schmid@ 1976; Heddle et al.@ 1983) (Figure I)@ Because of the speed and simplicity involved in UN evaluation@ MN analysis is often used as a sensitive@ yet less time-consuming alternative to classical metaphase analysis (Heddle et al.@ 1983). Although several forms of MN assessment have been introduced@ the best developed form of MN evaluation involves the scoring of MN n bone marrow polychromatic erythrocytes (PCEs) (reviewed by Heddle et al.@ 1983). The product of recent cell division@ these anucleated cells arc fully differentiated@ abundant@ and easily recognized. In addition@ these cells have a residency time in the bone marrow of approximately 24 hours@ providing Information about recent bone marrow damage. The mouse peripheral blood MN assay Introduced by MacGregor and coworkers offers increased simplicity and sensitivity while avoiding the tine constraints involved in the bone marrow MN assay. MacGregor's assay is based on assessing MN In PCEs and In normochromatic erythrocytes (NCEs) of the peripheral blood of animal species in which the spleen does not remove micronucleated erythrocytes from the circulating system [e.g.@ mouse@ Syrian golden hamster (MacGregor et al.@ 1980; Schlegal and MacGregor@ 1982; Tice et al.@ 198435- Analyzing MN frequency in peripheral blood cells permits niultiple sampling from the same animal under chronic exposure conditions and an ascertainment of both recent and past bone marrow damage. This latter advantage Is based on the fact that PCEs have a lifetime of '*-2 days in the peripheral blood and thus can indicate damage induced in the bone arrow 1to 3 days prior to sampling while NCEs In the mouse have a lifetime of .35 to 50 days and thus can be used to indicate damage induced in bone marrow for that time period. This approach Is especially useful because retrospective analyses can be conducted on peripheral blood sinears prepared for hematological evaluation of animals exposed chronically or sub chronically to test chemicals Using this approach@ Tice et al. (1984) obtained MN data on C57B1/6 mice exposed to 300 ppm benzene for 4@ 8@ and 16 weeks and sampled at 4@ 8@ and 16 weeks after termination of each benzene exposure which suggested that less bone marrow damage was being induced with Increasing exposure duration. However@ this experiment was poorly designed for this purpose and other possibilities related to significant alterations in hematopolesls could not be discounted. The experimental design utilized In the API study was much better qualified to address this question.