The effects of the calcium antagonist Nifedipine were investigated on isolated human myometrium and on uterine activity in healthy women during their first menstrual days. Nifedipine (0.01-1.0 μg/rnl) had a concentration-related inhibiting effect on spontaneous activity of the myometrial strips, and relaxed preparations contracted by potassium. In vivo, Nifedipine (20-30 mg) given orally, effectively and rapidly reduced uterine activity, decreasing both amplitude and frequency of uterine contractions, and reduced basal tone. A moderate increase in heart rate and a transient facial flushing were noted, but otherwise no side effects were observed. Calcium antagonists like Nifedipine represent a new approach to the problem of relaxing the myometrium, and might be an interesting therapeutic alternative in situations where inhibition of unwanted uterine activity is desirable.