Technical Abstract: At the onset of puberty, seminiferous tubules rapidly increase in diameter occupying a greater proportion of the testis with a consequent rapid increase in testicular size. The objective of the current studies was to evaluate the utility of ultrasonography to assess testicular diameter as a basis for ranking boars relative to their extent of pubertal development. In the initial study, prior to castration at 4, 5, 6, or 7 mo of age, testicular length and diameter were assessed by ultrasonography in 160 anesthetized boars. After castration, testes were weighed. Mean diameter of seminiferous tubules and percentage of the testis occupied by tubules were determined by histological evaluations of all testes. Testicular volume calculated for length and diameter correlated with testicular wt (P < 0.001; r >/= 0.78) within each of the four age groups. At 4 and 5 mo of age, testicular diameter correlated positively (P < 0.001) with diameter of seminiferous tubules; this relationship was not significant at older ages. From 4 to 7 mo of age, testicular wt increased proportionately at a greater rate than body wt. In two subsequent studies, testicular diameter determined by ultrasonography of conscious boars was highly correlated (r > 0.8) when assessed twice on the same day or when diameter of the right was compared to diameter of the left testis. Similarly, testicular diameter obtained initially at 92 d of age correlated positively (P < 0.001) with the diameter observed at older ages, but the magnitude of the relationship decreased as time between evaluations increased. These findings support ultrasonographic determination of testicular diameter during early pubertal development as a means to rank boars of similar chronological age for extent of pubertal development.