During the human menstrual cycle, serum inhibin concentrations fluctuate in a cyclic fashion. To examine the regulation of inhibin/activin beta(B) subunit gene expression in ovarian granulosa-luteal cells, the levels of beta(B) subunit mRNA were determined in primary cultures of human granulosa-luteal cells treated with gonadotrophins and protein kinase modulators. Granulosa cells were obtained from women undergoing an IVF programme. The cells were enzymatically dispersed, separated from red blood cells, and maintained in culture for 5--10 days before addition of different agents. Northern blot analysis with specific oligonucleotide probes was performed to study inhibin/activin beta(B) subunit mRNA levels. Both LH and FSH reduced the accumulation of beta(B) subunit mRNA in a dose-dependent manner. The protein kinase A activator, (Bu)(2)cAMP, and the protein kinase inhibitor staurosporine also inhibited beta(B) subunit mRNA expression dose-dependently. Activin A increased dose-dependently beta(B) subunit mRNA expression. Our study suggests that activin-induced and gonadotrophin-inhibited beta(B) subunit expression in granulosa cells might be key factors in the transition from inhibin B to inhibin A dominance during the menstrual cycle.