For the purpose of establishing clinical reference values, this paper reports results of ultrasonographic examination and measurement of accessory sex glands, ampullae, and the pelvic urethra of 102 mature, healthy breeding stallions (2-29 years of age) of various size types (7 Miniature Horses, 27 small ponies, 53 light horses and 15 heavy horses). Examinations were done per rectum in mostly unsedated stallions using an Aloka 210 scanner with a 7.5 MHz linear veterinary transrectal transducer (Corometrics Medical Systems, Inc., North Wallingford, CT, USA). Most measures of accessory sex glands, ampullae and the urethra were larger in horses of larger sizes. Except for vesicular glands, the majority of the measures for all glands were smaller for Miniature Horses and ponies than for light horses and heavy horses (P < 0.05). For vesicular glands, measures for heavy horses were greater than for those of other groups (P < 0.05). Measures were similar for Miniature Horses and ponies, and for light horses and heavy horses. For all measures, differences between left and right paired glands were not different (P > 0.10). The lumen diameter of vesicular glands and ampullae as well as prostate lobe thickness showed the greatest asymmetry. Although there were too few representatives of various breeds for statistical comparison, among the light horse breeds Arabian stallions had the smallest mean values for the majority of the measures. Among stallions, echogenic characteristics of accessory sex glands, particularly vesicular glands, varied widely, possibly related to variation in recent sexual activity. For some stallions, echogenic character, particularly that of vesicular glands, varied remarkably from left to right gland within stallions. For ampullae, there was also wide variation in lumen contents between stallions. These data are generally consistent with previous reports with smaller numbers of stallions, as well as consistent with in vitro measures in previous studies. The results provide useful clinical guidelines for size measures of accessory sex glands in horses.