Nine groups of bull calves (n = 5 to 6 per group) were castrated every 5 wk from 5 to 45 wk of age, and the stages of spermatogenesis were identified histologically. Prior to castration, the testes of each calf were examined by ultrasonography, and the pixel intensities of the parenchyma were quantitated. Testis ultrasonograms were also recorded every 2 wk from 10 bull calves between 2 and 40 wk of age. Blood samples were collected at weekly intervals until castration. There was an early transient rise in circulating LH concentrations between 4 and 25 wk of age, while circulating FSH concentrations were high initially but decreased between 14 and 30 wk of age. Circulating testosterone concentrations increased gradually from 6 to 35 wk of age and then rapidly to 42 wk of age. There was a progressive increase in the more mature cell types during spermatogenesis as the animals aged, with the most dramatic changes occurring between 15 and 45 wk of age. Outer seminiferous tubule diameter increased between 10 and 45 wk of age, with the most rapid increase occurring from 30 wk of age. Inner tubule diameter increased between 30 and 35 wk of age. The echogenicity of the testes (as determined by ultrasonography) increased between 20 and 40 wk of age. From these data we conclude that testis echogenicity increased during the most active phase of growth of the seminiferous tubules as more mature germ cells were produced. Cessation of the early rise in gonadotrophin secretion immediately preceded this active phase of testicular development. Testosterone secretion rose markedly with the production of mature spermatozoa.