Carbetocin, a long-acting oxytocin analog, was administered by intravenous and intramuscular injection to 40 women 24 to 48 hours postpartum. Intravenous injection of 8 to 30 micrograms produced a tetanic uterine contraction within 2 minutes, lasting about 6 minutes, followed by rhythmic contractions for a further 60 +/- 18 minutes. Intramuscular injection of 10 to 70 micrograms also produced tetanic contraction in less than 2 minutes, lasting about 11 minutes, and followed by rhythmic contractions for an additional 119 +/- 69 minutes. The prolonged duration of activity after intramuscular compared with the intravenous carbetocin was significant (p = 0.020). Carbetocin produced mild lower abdominal cramping in most patients and severe pain in three patients who received 50 or 100 micrograms intravenously or 70 micrograms intramuscularly. Approximately half of the patients also experienced flushing and warmth. Although carbetocin has not yet been studied immediately postpartum, its prolonged uterine activity suggests that carbetocin may offer advantages over oxytocin in management of the third stage of labor.