The respiratory depressant effect of marijuana was studied by means of carbon dioxide respiratory response curves in eight healthy male subjects participating in a study of the effects of a 90-day ad libitum intoxication to marijuana. Subjects were studied in the morning after refraining from marijuana use since the previous evening. Respiratory response curves were done before smoking and at 15, 30 and 60 minutes after smoking two 900-mg cigarettes containing 2.2% delta9-tetrahydrocannabinol. Studies were conducted 2 to 3, 5 to 6 and 8 to 9 weeks after onset of intoxication. There was a significant change from base-line controls, indicating respiratory depression following marijuana smoking. A significant change in the time-effect curve over these three study sessions, indicative of tolerance, was noted. The degree of pulse elevation also decreased over the three study sessions, but this decrease in effect was not statistically significant.