The perinatal distribution of butorphanol was demonstrated in relation to maternal-neonatal transfer and colostrum/milk excretion in obstetric patients. Parenteral butorphanol passed the placental barrier and was found in neonatal cord serum. The mean neonatal serum concentration of butorphanol was not different from the mean maternal serum concentration of butorphanol, following a 1 or 2 mg intramuscular dose. Butorphanol was detected in the milk of lactating women following oral and intramuscular administration. Serum and milk concentrations appeared to be parallel with time. This observation was confirmed by the constancy of the mean milk-to-serum concentration ratio (0.7 intramuscular, 1.9 oral). We calculated that 4 micrograms would be the maximum amount of butorphanol, which would be expected to be present in the full daily milk output (1 L) following administration four times a day of 2 mg intramuscular or of 8 mg oral doses. An oral dose of 4 micrograms to an infant weighing 4 kg corresponds to the negligible oral dose of 0.7 mg to a 70 kg adult. The demonstrated safety and efficacy of butorphanol as an obstetric analgesic and the characteristics of the maternal-neonatal transfer and milk excretion are indicative of the potential excellence of this agent for obstetric use.