The relation between marijuana consumption and the development of tolerance was investigated during a 31-day study. Volunteers with a history of moderate or heavy marijuana use were given access to one-gram (2.1% delta9 tetrahydrocannabinol [THC]) marijuana cigarettes during a 21-day smoking period. Both groups tended to increase consumption during this time. Heavy users averaged 5.7 cigarettes per day and indicated a progressive decline in ratings of intoxication and duration of pulse rate effect. Moderate users averaged 3.2 cigarettes per day but showed no changes in either of these reactions during this time. Results suggested that tolerance does not develop to the two most reliable indexes of marijuana intoxication unless heavy doses of delta9 THC are self-administered repeatedly. Also, the tendency to increase consumption during this time is not necessarily associated with the development of tolerance.