Pathologic changes in the endometrium of mares may be rated according to Kenney's method of classification. Category I endometrium contains healthy tissue with no or few widely scattered pathologic changes. At the opposite end, severe widespread pathologic changes are associated with category III. Uterine biopsy specimens were collected aseptically from 16 mares during the estrous and diestrous stages of the cycle. Pathologic changes were evaluated, using light microscopy, and endometrium was classified as Kenney's category I, II, or III. Endometrial tissue of category I (n = 5 mares in estrus; n = 3 in diestrus); category II (n = 3 in estrus; n = 4 in diestrus), and category III (n = 4 in estrus; n = 4 in diestrus) were processed for scanning electron microscopy (SEM). All specimens were fixed immediately after biopsy because it was found that numerous bleb-like projections were formed when fixation was delayed. Category I endometrium had normal glands, and fibrotic tissue was not observed by light microscopy. Scanning electron microscopy revealed numerous hexagonally shaped cells that were covered with many microvilli. Ciliated cells also were observed, and they contained long healthy cilia. Category II endometrium had 2 to 4 nests surrounded by collagen fibers. Of the 4 specimens, 3 had moderate leukocyte infiltration (59 +/- 14.8 WBC/4 high-power fields [450 x]). Scanning electron microscopy revealed some inflammatory changes with slight swelling of the cell surface. Several cells in category II endometrium lacked microvilli, but they were interdispersed among many healthy hexagonal cells. Many nests were seen in category III tissue, and 2 specimens had severe infiltration of WBC (232,264 cells/4 fields).(ABSTRACT TRUNCATED AT 250 WORDS)