A total of 23 mares were inseminated once within 0-6 h after clinical detection of ovulation, 14 with fresh and 9 with deep-frozen semen containing 0.1 x 10(9) to 4.7 x 10(9) motile spermatozoa. Within these two groups, the mares were slaughtered 2, 4 or 6 h after insemination and their genital tracts removed. The utero-tubal junction, isthmus and ampulla ipsilateral to the ovary in which ovulation occurred were flushed separately for sperm recovery. In 1 or 2 mares of each group, the uterine horn and corpus uteri, the cervix and vagina were also flushed. Tissue samples were collected from the contralateral oviduct and the other genital regions and prepared for scanning electron microscopy to show spermatozoa distribution in situ. Flushings were also prepared for scanning electron microscopy. There were no significant differences in the extent of sperm migration and in the number of spermatozoa in the different regions of the oviduct 2, 4 or 6 h after insemination of fresh or frozen semen. There was, however, a striking difference in sperm number within the time intervals examined; the numbers were greatest at 4 h after insemination. SEM of spermatozoa in the various regions of the oviducts failed to indicate any alterations to the sperm-head membranes that could be associated with sperm capacitation. The majority of spermatozoa found in the uterotubal junction, isthmus and ampulla showed morphological integrity.