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Two experiments were conducted on the effects of chronic administration of fenfluramine on behaviour and body weight in rats. In Experiment One the effects of 28 day chronic administration were studied. A dose related rapid weight loss was observed in treated subjects, with development of tolerance to the effects of the drug on body weight after 14 days administration. Observations of behaviour were made on days 1, 14 and 28 of chronic administration according to a “time sampling” procedure of behavioural categorisation. The incidence of some behavioural patterns varied significantly between observation days, although observations of control subjects were never significantly different. By the 28th day of administration tolerance to the behavioural effects of the drug had developed, no dose/response eifects being noted in contrast to the results for prior observation days. In Experiment Two confirmation of the development of behavioural tolerance was obtained. Abnormal, “stereotyped” behaviour induced by a very high dose of fenfluramine showed a much lower incidence in subjects that hadr eceived fenfluramine for 30 days than in saline controls. Attention is drawn to the difficulties inherent in describing psychotropic agents as either sedatives or stimulants. It is suggested that although fenfluramine is generally considered to be a sedative, stimulant effects may be observed after chronic administration of anorexic doses. Similarities between the effects of high doses of fenfluramine and amphetamine are described.