Granulosa cells were recovered from small (1-3 mm in diameter) and large (greater than 6 mm in diameter) preovulatory follicles or from follicles of early pregnant pigs (3-5 mm, Day 18). Incubation of these cells (5 x 10(5)) was carried out in a shaking water bath (40 degrees C) for 2 h with or without salbutamol (10(-5) M), isoprenaline (10(-5) M), propranolol (10(-5) M) and FSH (100 ng/ml). Isoprenaline significantly increased progesterone production (P less than 0.05) by granulosa cells of small follicles and large preovulatory follicles but not by granulosa cells of follicles from early pregnant pigs. After blocking the beta-adrenoceptor with propranolol the stimulatory effect of catecholamines was not observed. FSH alone stimulated progesterone production, particularly in granulosa cells of early pregnant pigs (P less than 0.05) but FSH plus catecholamine treatment did not have any effect on progesterone release. These results suggest that catecholamines may play a regulatory role in follicle maturation and this may differ between naturally cyclic and early pregnant animals.