SYNOPSIS - ABSTRACT Six healthy male human volunteers were exposed to carbon monoxide (CO) concentrations ranging from 1000 ppm to 45@600 ppm for periods of 10 minutes to 45 seconds. Carbon Monoxide was rapidly absorbed and the increase in percent carboxyhemoglobin (COHb) saturation in venous blood per liter of CO mixture inhaled could be accurately predicted by the equation: log (A% COHb/L) = 1. 036 log (ppm CO inhaled) - 4. 4793 The abrupt increase in carboxyhemoglobin concentration of 11@6 and 9. 1% saturation in two subjects produced the immediate onset of mild frontal headache@ The subject exposed to 35@ 600 ppm demonstrated slight sagging of the ST-segment of Lead II. This occurred 20 seconds after the exposure had started and persisted for 10 minutes post-exposure. Neither the spontaneous nor the evoked electrical activity of the brain exhibited significant changes which could be attributed to CO exposure over the range studied.