The effect of gradually increasing COHb saturation on human visuoperceptual and psychomotor performance was studied in 22 nonsmokers and 22 smokers. Each subject performed two sessions in randomized order, one during air breathing and the other during CO breathing on two separate days. Testing and COHb saturation measurement were repeated six times during each session. Gas breathing was between the test periods. The increase of COHb saturation up to 12--13 per cent units had no effect (p greater than 0,05) on perceptual speed and accuracy as measured by the Bourdon--Wiersma test. Finger tapping speed was also unaffected. Visual perception measured with critical flicker frequency (CFF) was sensitive to CO. The gradual increase in COHb saturation caused a linear decrease in CFF in the both groups. An increase of one per cent unit in COHb saturation caused significant decrease in CFF (p less than 0.001), when intraindividual changes were taken into account. During acute exposure to CO there was no difference in any test performance between the groups. During air preathing there was no difference in performance although there was a significant difference (p less than 0,001) in the COHb saturation levels. This negative finding might be due to adaptation of smokers to chronic exposure of CO because of smoking.