In vivo and in vitro evidence indicates that FSH is a primary regulator of testicular inhibin production. However, recent reports suggest that LH may also promote inhibin secretion in vivo. To investigate whether LH regulates inhibin subunit messenger RNA (mRNA) expression as well, we examined the effects of hypophysectomy and LH replacement on the testicular content of inhibin alpha and beta-B subunit mRNAs in sexually immature and adult rats. Twenty- and 60-day-old intact and hypophysectomized rats received saline or 0.25, 2.5, 25.0, or 250 micrograms ovine LH/100 g body wt sc, twice daily for 7 days beginning on the day after hypophysectomy (n = 5/group in three separate experiments). The inhibin subunit mRNA content of each testis was measured for statistical analysis by dot-blot hybridization, and experimental results were confirmed by northern analysis of poly(A) RNA from each sample. In immature animals, the testicular content of both inhibin subunit mRNAs was decreased after hypophysectomy; inhibin alpha and beta-B mRNA levels were decreased to 6.9 +/- 0.9% and 31.7 +/- 2.7% of intact control values, respectively. In adult animals, hypophysectomy resulted in a more modest decrease in inhibin alpha subunit mRNA per testis (44.9 +/- 3.1% of controls) but had no effect on beta-B subunit mRNA. Replacement of LH to immature hypophysectomized animals did not alter levels of mRNA for either inhibin subunit. However, in adults LH restored testicular inhibin alpha subunit mRNA content in testes of hypophysectomized animals to levels seen in intact, saline-treated control animals. LH replacement also slightly but consistently decreased testicular beta-B subunit mRNA content compared to levels seen in hypophysectomized, saline-treated rats. These results indicate that although the response of inhibin subunit mRNAs to pituitary input decreases as a function of sexual maturation, LH may play an important role in regulation of inhibin subunit mRNA expression in adult but not immature testes. The mechanism(s) of LH action on testicular inhibin subunit gene expression is currently unknown, but may involve either direct actions of LH on Leydig cell inhibin subunit mRNAs or indirect actions of interstitial cell factors on Sertoli cell inhibin subunit gene expression.