To test the possibility that cannabinoids are detectable following passive inhalation of Cannabis smoke the following study was performed. Five healthy volunteers who had previously never used Cannabis, passively inhaled Cannabis smoke for 30 min. Cannabis smoke was provided by other subjects smoking either marijuana or hashish cigarettes in a small closed car, containing approximately 1650 L of air. delta 9-Tetrahydrocannabinol (THC) could be detected in the blood of all passive smokers immediately after exposure in concentrations ranging from 1.3 to 6.3 ng/mL. At the same time total blood cannabinoid levels (assayed by radioimmunoassay [RIA] ) were higher than 13 ng/mL in four of the volunteers. Both THC and cannabinoid blood concentrations fell close to the cutoff limits of the respective assays during the following 2 h. Passive inhalation also resulted in the detection of cannabinoids in the urine by RIA and enzyme multiple immunoassay technique (EMIT) assays (above 13 and 20 ng/mL, respectively). It is concluded that the demonstration of cannabinoids in blood or urine is no unequivocal proof of active Cannabis smoking.