Growth hormone (GH), prolactin (PRL), cortisol, noradrenaline (NA) and euphoric responses to the mu opiate receptor agonist Fentanyl (FE) (0.2 mg/70 kg intravenously) were investigated in five healthy, drug-free, female volunteers at menstruation and at ovulation. The plasma GH response to FE was blunted at menstruation relative to the GH response at ovulation. Basal and maximal PRL concentrations were about 1.5 times higher at ovulation than at menstruation. Cortisol and NA concentrations as well as mood and behavioral responses were not influenced by the menstrual cycle. These results confirm earlier reports of a reduced GH response at menstruation to other pharmacological agents. They also demonstrate that the menstrual cycle is an important influence on neuroendocrine responses to opiate challenge.