Over two dozen Indian medicinal plants belonging to the families Leguminosae, Gramineae, and Malpighiaceae have been screened for potential psychodelic substances. The evaluation of only the indole bearing plants is reported here. Localization of about forty indole alkaloids, representing four broad structural types, viz., indole–3–alkylamines, carboxylated tryptamines, β–carbolines, and tetrahydro–β–carbolines, in the various individual parts of the plants has been made. The results indicate that the accumulation of two strongly polar type compounds, e. g., quaternary amines and the tert-amine oxides, takes place in the two extreme ends of the plants, the former type being present in the roots while the latter in the fruits. Pharmacological studies with the total alkaloid fractions and the individual entities have been conducted on two parameters: facilitation of serotonin response on isolated rat uterus and gross behavioral action in rats and mice. N,N–Dimethyltryptamine, 5–methoxy–N,N–dimethyltryptamine, 5–methoxy–N–methyltryptamine, bufotenine, harmine, (+)–tetrahydroharmine, and a new naturally occurring base, viz. (+)–5–methoxytetrahydroharman, elicited powerful behavioral effects in rats and mice. The effects, – hyperactivity, tremors, salivation, paralysis of hindlimbs, tapping of forelimbs, and convulsions, were almost completely antagonised by pretreatment of the animals with chlorpromazine. The alkaloids also reversed the reserpine–induced sedation in mice. The metabolism of indole–3–alkylamines and the role of aromatic β–carbolines as psychotropic agents is discussed.