The effect of an intravenous injection of 0.3 mg clenbuterol on myometrial activity, fetal heart rate and fetal outcome was studied in three groups of animals: six heifers in active labour at term, in which fetal oversize was diagnosed just before the drug was injected and whose calves were delivered by caesarean section within 50 minutes; four cows at the start of the expulsive stage of parturition which had been induced with flumethasone on day 270 of gestation; and in four parturient cows which had had electrodes implanted on the myometrium at least one week before calving was induced with flumethasone on day 270. Electrocardiograph electrodes were placed on the calf and an intrauterine pressure catheter was inserted between the calf and the uterine wall upon rupture of the amniotic membrane. Clenbuterol induced a significant decrease of myometrial activity for at least 20 minutes. Recovery was most rapid in the heifers in which an obstetrical examination had taken place during active labour. There was no significant effect on basal fetal heart rate but decreases in heart rate were absent as long as uterine contractions were inhibited. The fetal outcome, judged either before or at birth by blood pH, base-excess and PCO2 and by a clinical examination, was not adversely affected.