The biochemical and the pharmacological effects of beta-carbolines in animals and man are reviewed. Biochemical studies have revealed beta-carbolines' several actions, including inhibition of MAO-A, competitive inhibition of 5-HT uptake, general inhibition of Na+ dependent transports, binding to benzodiazepine and opiate receptors and probable action on dopamine receptors, which may all participate to a variable degree in the actions of different beta-carbolines. Many early in vivo studies, however, have concentrated on some harmala alkaloids, particularly harmaline or harmine. The effects of beta-carbolines in man are compared in this review with the symptoms of alcohol withdrawal. However, no human studies have been reported with those tetrahydro-beta-carbolines shown to occur in human body in normal conditions or after alcohol intake. To prove any connections of beta-carbolines with the withdrawal syndromes or other neurological and psychiatric diseases means that these compounds have to be shown to have abnormal central nervous system concentrations in these diseases. The physiological role of beta-carbolines has yet to be shown. They may act as neuromodulators and some, especially 6-methoxytetrahydro-beta-carboline, may have an endocrinological function. It has been suggested that some beta-carbolines act as the physiological ligands (agonists) of the benzodiazepine receptors, but the physiological beta-carbolines so far known seem to have other effects, such as the inhibition of MAO-A or 5-HT uptake in low concentrations.