More than 70 male chronic marihuana users were studied under research ward conditions for a 31-day period consisting of 5 days of baseline assessment, 21 days of marihuana availability, and 5 days of postsmoking assessment. Biological findings were for the most part within normal limits. Blood chemistry studies showed no abnormalities. Plasma testosterone levels for all subjects at all times during the study were well within normal limits, suggesting that previous reports of testosterone suppression by marihuana may have statistical but not biologic singificance. A significant reduction in baseline vital capacity was observed in six subjects, a compromise in pulmonary function similar to that characteristically produced by chronic inhalation of substances, such as tobacco smoke, which are irritants to the lung. Increased caloric intake and weight gain occurred in virtually all subjects and were causally related to marihuana smoking. The weight gain may be attributable to water retention as well as increased caloric intake. Behavioral findings indicated that no uniform alteration in mood is produced by marihuana smoking--all subjects reported becoming "high", but experienced no consistent degree of euphoria. Mood changes which occurred were relatively mild and were largely dependent on group determinants rather than individual experience. No relationship could be established between marihuana use and motivation to work or engage in socially desirable activities. Evidence that pulmonary function may be compromised as a function of marihuana smoking suggests the need for alerting individuals to this potential health hazard. As was true of cigarette smoking, the eventual public health consequences of marihuana use may become apparent only after large numbers of individuals have smoked marihuana for two or three decades.