The present study examined the effects of systematic manipulation of breathhold duration (0 and 20 s) on the physiological and subjective response to active (M; 2.3% delta-9-THC) and placebo (P; 0.0% delta-9-THC) marijuana in a group of ten regular marijuana smokers. During the eight-session experiment, subjects were exposed twice to each of four experimental conditions (P0, P20, M0, M20), scheduled according to a randomized block design. A controlled smoking procedure was used in which the number of puffs and puff volume were held constant. Expired-air carbon monoxide (CO) levels were used to monitor smoke intake. Breathhold duration affected CO absorption; significantly more CO was absorbed from both P and M smoke after 20 s of breathholding (mean CO boost = 6.9 ppm) than after no breathholding (mean = 4.4 ppm). Heart rate was minimally affected by the breathhold manipulation. Effects of marijuana on mood were not consistently affected by breathhold duration. The results confirm previous findings that prolonged breathholding does not substantially enhance the effects of inhaled marijuana smoke.