Twenty young, healthy male subjects exposed to a single exposure of 650 ppm carbon monoxide (CO) for 45 minutes increased their mean carboxyhemoglobin (COHb) by 7.61%. Another twenty subjects exposed to 950 ppm CO for 45 minutes increased their COHb by 11.22%. Before and after exposures all subjects were given tests for depth perception, visual discrimination for brightness, reaction time to a visual stimulus, and flicker fusion discrimination. The only test showing significant mean decrement after exposure was the reaction time test (P = 0.005 for one exposed group; P = 0.006 for the other). However the decrement produced by each group was essentially the same (F = 0.08).