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The effect of aging on the pituitary and testicular response to trophic hormones was studied in apparently healthy C57BL/6J male mice, aged 12 and 28 months.No age-related differences were observed in basal (non-fasting) plasma levels of LH, FSH, TSH, PRL, or GH or in plasma LH and TSH after stimulation with LH releasing hormone (LHRH) or TSH releasing hormone, respectively. The output of LH and FSH by 8 day cultures of pituitary cells during a 4 h stimulation by LHRH was also not related to donor age.The postcastration elevation of LH in plasma tended to be smaller in 28 than in 12 month old mice, although no age differences were observed in FSH levels.The increase of plasma testosterone (T) 30 min after injection of LH was 30% smaller in 28 month old mice but the difference was not statistically significant. Moreover, no age-related differences were found in the in vitro testicular output of T in response to LH.The maintenance of certain responses to trophic hormones by the pituitary and testes during aging in C57BL/6J mice contrasts with Wistar and LongEvans rats, in which major age-related impairments were reported for corresponding measurements. Thus, there may be species differences in the effect of aging on pituitary and testicular function in rodents. (Endocrinology 101: 1310(Endocrinology 101: , 1977 T HE EFFECT of aging on testicular functions does not show a consistent pattern in laboratory rodents and humans. Plasma testosterone (T) decreases markedly after mid-life in most men (1,2) and in Wistar (3) and Long-Evans (4) rats. However, plasma T is maintained during the average (26-30 month) lifespan of C57BL/6J (5,6) and DBA/2J (6) mice. Since plasma T is decreased in C57BL/6J mice with gross pathologic lesions (5), age-related diseases, as well as the genotype, may contribute to