Carbon monoxide (CO) was evaluated for its effects on the operant behavior of laboratory rats and juvenile baboons. For the rat studies the procedures included a 2-min variable interval (2-min VI) or fixed interval (2-min FI) schedule of reinforcement, a 60:1 fixed ratio (FR) schedule and a multiple fixed ratio, fixed interval (FR:FI) schedule. Carbon monoxide concentrations of 50-200 ppm for 2-hr exposure periods produced minimal effects on VI rates. Response rates were increased for all animals somewhere in the concentration range of 25-200 ppm. Response rates were depressed at 500 ppm CO. Behavioral effects at 50 ppm CO were not correlated with blood CO levels. Data averaged for grops of three animals show FI rate increased slightly above controls at 50 and 100 ppm CO and decreased at 200 ppm. FR rate increased at 50 ppm and decreased at 100 and 200 ppm. For the baboon studies, a delayed match-to-sample discrimination task was used. The effects of CO (6 hr per day over a 5-day period) on the discrimination task was limited to an occasional mistake by an animal. CO did produce a slowing of the animals' response times as well as an increase of extra responses during the delay intervals. The changes in extra responses were dose related.