Butorphanol tartrate 1 mg and 2 mg were compared in 80 normal mothers at term in a double-blind study with meperidine hydrochloride 40 mg and 80 mg for the relief of pain in labour. Butorphanol was found to be as effective as meperidine in relieving pain in labour. The foetal condition, as measured by ECG monitoring, Apgar scores, time to sustained respiration, umbilical venous H+ (pH) and PCO2, and a general nursery survey were comparable for meperidine and butorphanol. No psychomimetic phenomena were seen. Assays indicated that both butorphanol and meperidine crossed the placenta. The mean concentration of butorphanol in neonatal serum was 0.84 times maternal serum at 1.5 to 3.5 hours after intramuscular administration of a single or two successive doses of butorphanol 1 mg or 2 mg to the mother. The mean concentrations for meperidine in neonatal serum was 0.89 times maternal serum at 0.85 to 3.6 hours after intramuscular administration of meperidine 40 mg or 80 mg to the mother. Neither analgesic caused severe depression of the infant except for one meperidine-treated case.