"INTRODUCTION A comprehensive@ critical review of the world literature on the toxicity of cyanides and related compounds to fish (Doudoroff@ 1976) was completed in 1975. It contained much information about the acute@ lethal toxicity of both simple and complex cyanides@ but relatively little about the sublethal effects of chronic cyanide poisoning. Since its publication@ numerous reports of additional research into the toxicity to fish of free cyanide (not the metallocyanide complexes) have appeared@ many of which deal mainly with sublethal effects. Also@ a few recent@ pertinent publications and theses of much interest@ as well as some earlier papers of minor importance@ had been overlooked when the above-mentioned review was prepared. Therefore@ there is a need for an additional review of the very recent literature. This review covers only publications and theses that appeared after the year 1972 and were not cited by Doudoroff (1976). Although a few other review papers are cited@ only original sources of new information are fully considered. References to some early@ pertinent papers not mentioned in my reviews can be found in cited@ recent publications (e.g.@ Leduc@ 1978). It is appropriate to mention here that a reference to the important paper by Herbert and Merkens (1952)@ which was mentioned repeatedly by Doudoroff (1976)@ unfortunately was inadvertently omitted from the list of references at the end of the review. It can be found in the ""Literature Cited"" section of the present review and should be inserted where it originally belonged. With one exception@ all of the recent experiments reported here were performed with constant-flow apparatus for continuous renewal of the cyanide solutions. The type of test is not usually mentioned@ therefore@ in describing the experiments. Actual concentrations of free cyanide in the experimental solutions have usually been determined at regular intervals by chemical analysis. The concentrations reported are either those of molecular HCN only@ derived by calculation from the determined levels of all free cyanide present@ or are levels of all free cyanide (HCN and CN- ion) expressed as the CN- or (more frequently) the HCN equivalents. Because most of the experiments were performed at pH levels not much above 8.0@ at which little of the free cyanide is present in the ionic form@ and in view of the small difference in molecular weight between HCN and the CN radical@ the distinction between the concentrations reported in these different ways generally is of little importance. An exception is a special study of the relative toxicity of the two forms of free cyanide@ which necessarily included some tests at high pH levels. In the comprehensive research report of Smith et al. (1979)@ many data were presented that had previously been published by Kimball et al. (1978)@ Koenst et al. (1977)@ Lind et al. (1977)@ or Smith et al. (1978). In this review@ reference-rs-made only to the latter-(earlier) publications when reporting data that are presented there. The report of Smith et al. (1979) is cited only when some information is reported that was not to be found in the earlier papers mentioned above. The cooperation of Dr. S. J. Broderius@ Mr. D. G. Dixon@ and Dr. G. Leduc@ who kindly supplied useful research reports and data in advance of their publication or general availability and otherwise assisted in the preparation of this review@ is very gratefully acknowledged."