This report describes a clinical trial in which pericyazine and chlorpromazine were used with 45 disturbed patients resident in a hospital for the mentally subnormal. The patients, who comprised a clinically heterogeneous sample, were randomly allocated by the hospital pharmacist to two major groups; in one, half the patients were given pericyazine for a 12-week period and an inert preparation for a further 12-week period while the remaining patients had the inert preparation during the first period and pericyazine during the second. In the other group, which was given chlorpromazine, the same procedure was followed. Prior to the trial proper, there was a six-week period during which all drugs except anti-convulsants were tapered off. All preparations were in syrup form and were indistinguishable from each other by taste or smell. Only the pharmacist was aware of what syrup a given patient received.