1 The pharmacological potencies of the resins from three different samples of Brazilian marihuana (A, B and C) were determined through corneal areflexia in rabbits, decrease of spontaneous motor activity and induction of catatonia in mice, and decrease of rope climbing performance of rats.2 The Delta(9)-tetrahydrocannabinol (Delta(9) THC) content of the marihuanas, measured by gas chromatography, was 0.82, 2.02 and 0.52%, respectively, for samples A, B and C. Approximately 2% cannabinol was present in samples A and B whereas the content of cannabidiol was approximately 0.1%.3 The petroleum ether extraction of the samples A, B and C yielded, respectively, 12.06, 14.56 and 4.26% of resin.4 In all animal tests resin B was nearly twice as active as resin A, whereas C was the weakest.5 The smoke of the marihuana samples was inhaled by 33 human subjects, under a double-blind standardized procedure. Pulse rate, a time production task and an evaluation of psychological effects were recorded.6 The smoke of 250 mg of sample B provoked disruption of the time production task, increased pulse rate, and induced strong psychological reactions in four of the six subjects who received it. Similar effects, although slightly smaller, were obtained with 500 mg of sample A. On the other hand, 500 mg of sample C did not differ from placebo.7 It is suggested that it is possible by means of animal tests to predict the potency of a marihuana sample in man.8 In parallel experiments, Delta(9)-THC was administered to other human subjects and to laboratory animals in a manner similar to that in which the marihuana samples were administered.9 Comparison of the results between the marihuanas and Delta(9)-THC showed that in man and in the laboratory animals marihuanas A and B induced effects two to four times greater than expected from their Delta(9)-THC content.10 It is suggested that there may be potentiation of the effects of Delta(9)-THC by other substances present in these marihuana samples.