The effects of exogenous growth hormone (GH) and FSH on development of the testes in intact prepubertal boars was investigated. Twenty-four boars received one of four daily treatments from 8 through 40 days of age: 1) 90 μg porcine (p) GH/kg body weight (BW), 2) 100 μg pFSH/kg BW, 3) GH + FSH, or 4) vehicle only (control). Plasma testosterone levels, measured at 10-day intervals, were similar among groups of boars throughout the study. Body weights among groups were similar during treatment, and testicular weight between treatment groups did not differ at castration (100 days of age). However, total length of the seminiferous tubule per testis in FSH-treated boars was 59% greater than in GH-treated animals (2705 vs. 1704 m; p < 0.05). Diameter of the tubule in GH-treated boars was 56% greater than in FSH- treated boars (p = 0.03). Relative mass of spermatocytes and spermatids in GH-treated animals exceeded that in controls by 2.5-fold and that in FSH boars by 75-fold (p = 0.05). There were no differences in effects of GH + FSH treatment as compared to control treatment; none of the treatments affected any interstitial tissue parameter measured. These results suggest that exogenous FSH had a mitogenic effect on Sertoli cells while delaying tubular maturity, whereas exogenous GH promoted tubular and Sertoli cell maturation, defined as increased Sertoli cell size, lumen formation, and onset of spermatogenesis.