In response to the partial serotonin agonist meta-chlorophenylpiperazine (metaCPP), patients with obsessive-compulsive disorder have been reported to exhibit an increase in obsessive symptoms and a diminished release of prolactin and/or cortisol compared to controls. We examined the response to metaCPP of 10 patients with anorexia nervosa, before and after weight gain, and of eight healthy controls. Prior to weight gain, the patients exhibited a greater behavioral response to metaCPP than the controls and diminished prolactin and cortisol responses. Only the prolactin abnormality persisted after weight gain. These observations suggest that, in patients with anorexia nervosa, weight loss is associated with the development of neurobiological disturbances which are in some ways similar to those of obsessive-compulsive disorder.