The improvement shown by a group of severely subnormal patients after treatment with prothipendyl (Tolnate) was the reason for conducting a double-blind trial to assess the effects of the drug, while attempts were made to control the environment. The preliminary group studied consisted of 35 patients of both sexes, of all ages, who all exhibited pronounced behaviour disorders and presented acute problems in nursing and management. None had responded adequately to the more widely used tranquillizers and sedatives, but all except two patients with superadded depression, responded to prothipendyl in some measure without side effects.