Four concentrations of nifedipine (AdalatR, Bayer) were infused into 25 pregnant sheep of 123-140 days of gestation (term, 147 days) and the effects on the ewe and the foetus have been studied. At all doses of nifedipine infused, maternal diastolic pressure fell by about 15% and maternal heart rate increased by 33%. There was no change in blood gases or pH. Uterine activity, as measured by uterine electromyographic recordings, was reduced due to an increase in the interval between periods of activity. The duration of a burst of activity remained unaffected. The effects of nifedipine on the foetus, were similar. Mean foetal arterial pressure fell by 4-5 mmHg and heart rate rose by 15 to 50%, both changes being maintained for the duration of the infusion and the increased heart rate for longer. The electrocorticogram of the foetal sheep was unaffected by nifedipine. The effects on foetal breathing movements were small. At the concentration of 5 micrograms/kg/min for either 2 or 4 hours the breathing pattern changed so that the episodes of breathing were shorter and more frequent. The total amount of breathing per hour was unaffected. Control infusion of ethanol had little effect on the ewe except for a significant increase in lactate production. In the foetus breathing was reduced at the highest concentration used.