The short-term action of PMSG on the population of growing follicles in cattle was studied using histological methods. On Day 7 of a synchronized oestrous cycle 10 Friesian heifers were unilaterally ovariectomized. The remaining ovary was immediately stimulated by an injection of PMSG (2000 i.u.) and was removed 48 h after the preovulatory discharge of LH. Control animals did not receive any injection of PMSG. In all ovaries, follicles greater than 70 micron diameter were counted, measured and checked for atresia. The mitotic index in granulosa cells of follicles of different sizes was estimated in both ovaries of all the PMSG-injected animals. Unilateral ovariectomy alone had no significant effect on follicular populations. In the interval between PMSG injection and removal of the second ovary (148 +/- 22.7 h), PMSG significantly increased the number of normal preantral follicles but did not change the number of normal antral follicles. The mitotic index doubled in preantral and early antral follicles but remained unchanged in large antral follicles. PMSG stimulated slightly the growth of the antrum in large antral follicles but did not stimulate its formation in preantral follicles. The incidence of atresia among antral follicles, particularly the largest ones (diam. greater than 1.7 mm), was significantly reduced after PMSG, suggesting some 'rescue' of follicles from atresia.