CD31-positive blood vessels increased in human ovarian cortical tissues cultured with relaxin than in tissues cultured without relaxin. The number of vWF-positive vessels counted in rat ovaries after relaxin injection (40.5 +/- 4.3) significantly increased compared to that in saline-injected rats (24.3 +/- 3.0). The proportion of primordial follicles was significantly decreased, that of primary follicles was significantly increased at 16 h, and that of early antral follicles was significantly increased at 24 h in rats after relaxin injection. These results suggest that relaxin may contribute to follicle development through the mechanism of angiogenesis in the ovary.