In the previous study, using carbon tracer, we reported the ultrastructural evidences which suggested the increased capillary permeability of preovulatory rabbit ovarian follicles. In the present study, another tracer, horseradish peroxidase (HRP 50 A in diameter, mw. 40,000), being smaller than carbon particle, was used to examine the permeability of perifollicular capillaries. HRP left capillaries and appeared in the follicular cavity immediately after the injection into aorta, through all stages of follicular development. In the capillaries, the narrow interendothelial clefts (about 100-200 A in width) were observed through preovulatory stages. Fenestrations were noted at 3,4,6 hours after the hCG injection, and interendothelial gaps (about 1,000-10,000 A in width) were seen at 10, 12 hours. HRP passed through these structures and infiltrated the capillary basal lamina and the basal lamina of the follicle and went through the intercellular spaces between granulosa cells. Therefore, the increased permeability of perifollicular capillaries, just prior to ovulation, were not so obvious as when carbon particles were used. On the other hand, carbon particles, injected via marginal ear vein, were incorporated in the endothelial vesicles 15 min. after its administration and retained beneath the basal lamina outside the follicle within 60 min. These observations indicated that "blood follicle barrier" consisting of capillary endothelium, basal lamina, basal lamina of the follicle and granulosa layer played an inhibitory role to large molecules but not to small molecules.