Correlation between serum testosterone concentration and morphometric findings from ultrasonography of the accessory sex glands in peripubertal colts was investigated during pubertal development. Nineteen colts of initial age ranging from 5 to 12 months were monitored over a 13-month period. Serum testosterone concentration was determined on a biweekly basis, and accessory sex gland development was ultrasonographically monitored once a month. Notwithstanding individual variation, there was significant correlation (r = 0.913; P < 0.01) between increasing serum testosterone concentration and the onset of developmental changes involving the accessory sex glands. As colts entered their 2-year-old year with relatively immature reproductive tracts, compared with mature stallions, there was still a significant seasonal effect on serum testosterone concentration and accessory sex gland measurements (P < 0.05). Ultrasonography was confirmed as a valuable noninvasive method of monitoring and assessing peripubertal accessory sex gland development in colts.