Ten unmedicated female inpatients with major depression & 10 healthy volunteer women were given an intravenous injection of 0.1mg fentanyl at 9:00 AM & 9:00 PM on different days. The prolactin secretory response to this opioid agonist was investigated for 1 h with serial blood sampling. Repeated measures Analysis of Variance yielded a significant effect of fentanyl administration on prolactin secretion, & there were elevated hormone responses in the evening. No group difference was seen between healthy volunteers & depressed patients, but four of the depressives showed the most blunted response. Three of these low responders committed suicide within 1 year. (13 refs)