Histological sections prepared from cortical parts of 25 bovine ovaries were used to study initiation of follicle growth in vivo. Small follicles were measured and characterized. Initiation of follicle growth consisted of two distinct consecutive phases. The first phase was characterized by transformation of granulosa cells from a flattened to a cuboidal shape and by their proliferation. In the second phase an increase in the number of granulosa cells was accompanied by a rapid increase in the size of the oocyte. Oocytes commenced growth when there were at least 40 granulosa cells in the largest cross-section (fourth generation of follicle cells). The oocyte diameter increased from 29.74 +/- 0.30 microns (mean +/- SEM) in primordial follicles to 92.90 +/- 4.50 microns in small antral follicles. The zona pellucida first appeared as an island of periodic acid-Schiff positive material in small preantral follicles, but formed a complete ring around the oocyte when the late preantral stage was reached. Organ culture of ovarian cortical explants was used to study initiation of follicle growth in vitro. Within 2 days of culture most of the primordial follicles entered the growth phase: granulosa cells changed from a flattened to a cuboidal shape and entered S-phase as demonstrated by autoradiography after [3H]thymidine incorporation. On day 2, 48.6% of follicles were labelled compared with 3% on day 0. Follicle growth started in the absence of gonadotrophins, in the serum-free medium, confirming the notion that gonadotrophins are not essential for this process. The culture system used here will be helpful in the study of the involvement of putative factor(s) in the initiation of follicle growth in large domestic animals.