Within 45 minutes after mating in the rabbit, numerous heterophil leukocytes adhere to the endothelium of venules in the vagina. Initial association appears to occur via small protuberances from the leukocyte which fit into small indentions in the endothelial cell. Following adherence, leukocytes flatten and pass between endothelial cells. A regular intercellular space separates the leukocyte from the endothelial cells. Leukocytes subsequently migrate through the connective tissue to the epithelium. By three hours postcoitus, the region beneath the basement lamina of the vaginal epithelium is crowded with numerous juxtaposed leukocytes. Leukocytes subsequently migrate between epithelial cells into the vaginal lumen where they actively engulf spermatozoa. Spermatozoa appear to be ingested head first. Numerous small filaments are observed in the leukocyte cytoplasm in the region adjacent to the sperm head. Degranulation of azurophyl granules follows sperm uptake. The leu kocyte response can be elicited either by spermatozoa (from the epididymis) or by semen (from vasectomized bucks), but is not elicited by ovulation (with human chorionic gonadotropin). It is suggested that the response may be initiated because the vagina does not distinguish between semen, spermatozoa and bacterial infection.