The effectiveness of pericyazine in severe behavioural disorders was evaluated in 15 profoundly and severely retarded children. Pericyazine provided significant improvement in such parameters as co-operation, temper, purposeless activities, hyperactivity, communication and mood. It proved to be statistically superior to the minor tranquillizers in improving co-operation and helpfulness, temper, mood, the understanding of commands and table manners, and in reducing self-abusiveness and abusiveness to staff. The safety of this agent was confirmed and photosensitivity was not found to be associated with its use.