The function of the hypothalamic-pituitary-testicular axis was evaluated in rats fed a low protein diet for 4 weeks beginning at 21 days of age. Compared to control, the low protein group had decreased seminal vesicle and prostate weights as well as decreased testicular testosterone output in vitro, although serum testosterone was not different. The low protein group showed no consistent alterations in serum LH (basal, post-LHRH, and postcastration) compared to control although serum FSH (basal and post-LHRH) was lower in the low protein group. Despite this lower basal FSH, the low protein group had supranormal serum FSH after castration. Seminiferous tubule diameter and testicular histology were normal in the low protein group although testicular androgen-binding protein was absent. Testicular androgen-binding protein was also undetectable in a modestly food-restricted control group which had normal testicular size, testicular histology, androgen output, and serum FSH. This finding suggests that loss of testicular androgen-binding protein may be a sensitive sign of undernutrition. We conclude that rats fed a low protein diet have hypoandrogenism, normal testicular histology, and supranormal FSH after castration despite subnormal basal FSH. The latter combination suggests overproduction of an FSH inhibitor of testicular origin.