Behavioral effects of carbon monoxide (CO) were studied in rats during 90 min exposures to concentrations ranging from 250 to 850 parts-per-million (PPM). The animals performed on a multiple fixed-ratio differential-reinforcement-of-low rate (MULT FR 30 DRL 18) schedule of food reinforcement. Periodically, the animals were individually exposed to either air or CO for 30 min periods prior to as well as during 60 min test sessions. Carboxyhemoglobin measurements indicated equilibration after 60 min of exposure. Concentrations of 650 PPM or higher produced responses rate reductions. The decreased response rates were due primarily to abrupt cessation of responding during CO exposures. Response patterning in both FR and DRL components remained intact until responding ceased. The accuracy of responding in the DRL component as measured by the distribution of inter-response-time (IRT) intervals was not systematically affected by CO exposure.