The purpose of this study was to examine the relative roles of FSH and LH in stimulating testicular inhibin secretion in the male rhesus monkey. Recombinant human (rh) FSH and rhCG were used as the gonadotropic stimuli, and juvenile rhesus monkeys, in which the endocrine activity of the pituitary-testicular axis was being driven in an adult manner with an intermittent i.v. GnRH infusion, were studied. Immunoactive inhibin levels were measured by the Monash RIA. Initiation of an intermittent i.v. infusion of rhFSH (10 IU every 3 h) resulted, after a delay of 5-6 h, in a progressive increase in the concentrations of immunoactive inhibin, which achieved, after 48 h of stimulation, a value twice that observed during vehicle treatment. Gel filtration chromatography revealed that the FSH-induced elevation in immunoactive inhibin was the result of an increase in three distinct mol wt fractions: peak I (100 kDa), peak II (50-60 kDa), and peak III (31 kDa). Although peak III accounted for most of the inhibin immunoactivity in vehicle-treated animals, peaks I and II were most responsive to FSH stimulation. Application of recently developed enzyme-linked immunosorbent assays for inhibin B and pro-alpha-C-related peptides provided additional insights into the nature of the FSH-sensitive forms of circulating immunoactive inhibin. Most notably, the 31-kDa fraction (peak III) was comprised of inhibin B and pro-alpha-C. In contrast to FSH stimulation, an intermittent infusion of rhCG (40 IU every 3 h), which markedly elevated testicular testosterone secretion, failed to increase immunoactive inhibin concentrations. These findings indicate that various forms of immunoactive inhibin are present in the circulation of the rhesus monkey, and that in this species, FSH is the principal stimulus of the secretion of testicular inhibins, including inhibin B. Additionally, they further underline the importance of the FSH-inhibin feedback loop in governing testicular function in primates.