Nocturnal hormone profiles were measured in patients with schizophrenia with predominantly negative symptoms both under drug-free baseline conditions and after subchronic administration of the atypical antipsychotic olanzapine, with the aim of characterizing its pharmacological properties on the neuroendocrine level. The following hormones were studied in the sleep laboratory under polysomnographic control: adrenocorticotrophic hormone, cortisol, growth hormone (GH), prolactin, testosterone, and melatonin. Blood samples were taken at regular time intervals over the night, and serum concentrations of the hormones were determined. Ten patients completed the study, two of them were excluded from analysis due to incomplete hormone profiles. The dynamics of baseline nocturnal hormone secretion were similar to the patterns known from healthy subjects. After the treatment period of about 4 weeks, hypothalamic-pituitary-adrenal axis activity was reduced with decreased cortisol plasma levels compared to baseline conditions. Olanzapine induced a moderate prolactin elevation. The characteristic GH peak around sleep onset, clearly present under baseline conditions, was markedly reduced after treatment. Testosterone and melatonin secretion were not significantly altered. In conclusion, although interpretation is difficult in some cases due to interference with indirect effects of olanzapine administration and the consequences of the clinical course of the underlying schizophrenic disorder, the neuroendocrine findings are consistent with the receptor-binding profile of olanzapine where, beside the D(2) antagonism, the antiserotonergic properties are most important.