The purpose of this study was to measure the concentration of fentanyl in human colostrum after intravenous administration of an analgesic dose. Thirteen healthy women were given fentanyl 2 micrograms.kg-1 for analgesic supplementation during either Caesarean section or postpartum tubal ligation. Serum and colostrum were collected for 45 min, two, four, six, eight, and ten hours following administration of the drug. Radioimmunoassay showed that colostrum fentanyl concentrations were greatest at 45 min, the initial sampling time, reaching 0.40 +/- 0.059 ng.ml-1, but were virtually undetectable ten hours later. Fentanyl concentrations were always higher in colostrum than in serum. This concluded that with these small concentrations and fentanyl's low oral bioavailability, intravenous fentanyl analgesia may be used safely in breast-feeding women.