Three studies in which exposure to marihuana smoke was progressively increased were conducted. In each, four experienced marihuana users smoked marihuana cigarettes in the presence of two nonsmoking subjects. They were confined in a closed environment for 1 hr after the smoking began. All urine voided by the nonsmoking subjects was collected separately for 24 hr and analyzed for the presence of cannabinoids by the enzyme multiplied radioimmunoassay technique (EMIT) cannabinoid assay. Blood samples were drawn from one of the nonsmoking subjects in one of the studies, and delta 9-tetrahydrocannabinol (THC) plasma concentrations were determined by radioimmunoassay. The results obtained indicate that of 80 urine samples collected in the three studies, the drug levels in only two slightly exceeded the value of 20 ng/ml on the calibrator of the EMIT assay. Minute but detectable concentrations were present in the plasma of the one subject from whom blood samples were obtained. In this subject intravenous infusion of THC at the rate of 3.2 micrograms/min for 1 hr resulted in THC plasma concentrations identical to those during his passive inhalation of marihuana smoke.