The uterine function and reproductive efficiency of 31 nonlactating pony mares were compared for two age groups: young (5 to 7 years, n=9) and old (>/=15 years, n=22). For pregnant mares, differences between age groups were not significant for the diameter of the largest follicle, cross-sectional area of the corpus luteum, growth profile of the embryonic vesicle or embryo mobility characteristics. Uterine contractility scores were lower (P<0.05), day of fixation of the embryonic vesicle was later (P<0.05), and uterine tone tended (P<0.10) to be lower in the old than the young mares. Endometrial biopsies in old mares had more (P<0.05) inflammatory cell infiltrations, more (P<0.05) fibrotic changes, and less dense (P<0.05) endometrial glands than in young mares. Ultrasonically detected intrauterine fluid collections were more extensive (P<0.05) in the old than the young mares. The pregnancy rate on Day 12 (Day 0=ovulation) was lower (P<0.05) and embryo-loss rate (Days 12 to 39) was greater (P<0.05) in old (32 and 62%, respectively) than in young (100 and 11%, respectively) mares. The results confirmed previous reports that old age was associated with increased endometrial inflammation, reduced pregnancy rate and increased embryo-loss rate. The results also indicated that uterine contractility and uterine tone were reduced and the fixation of the embryonic vesicle occurred later in old than in young mares.