It seems to be safe to conclude from the presented data that a first exposure to some hallucinogenic drugs does affect the metabolic machinery of the human organism in the direction of increased metabolism. Enzyme induction comes immediately to our mind as a possibility, but the data do not allow us to propose this as the only possible explanation. Whether or not this biochemical change is related to some long term psychological and physiological effects cannot be ascertained. We can only suggest that the clinical significance of the "psychedelic afterglow" (Kurland and Unger, 1968), the increased reactivity of chronic " acid heads" to low intensity light stimuli (Blacker et a/., 1968) and the increased psychological responses to marihuana after the first exposure (Weil, 1968) seem to be strong enough impetus for a continued search for biochemical and physiological correlates of long lasting effects of hallucinogenic drugs.