The role of transforming growth factors (TGFs) in the acquisition of granulosa cell aromatase activity was investigated in vitro in a primary culture of granulosa cells harvested from immature, diethylstilbestrol-treated rats. Basal aromatase activity, as assessed by the generation of radioimmunoassayable estrogen, was negligible, remaining unaffected by treatment with either TGF alpha or TGF beta applied by themselves at the 10 ng/ml dose level. Whereas treatment with FSH produced a substantial increase in the extent of aromatization, concurrent treatment with TGF beta (0.01-10 ng/ml) resulted in dose-dependent augmentation of the FSH effect with an apparent median effective dose of 224 +/- (SE) 32 pg/ml (ca. 9 pM), and a maximal effect 3.6-fold greater than that induced by FSH alone. In contrast, concomitant treatment with TGF alpha (0.01-10 ng/ml) resulted in dose-dependent attenuation of FSH action with an apparent median inhibitory dose of 330 +/- (SE) 40 pg/ml (ca. 60 pM), and a maximal inhibitory effect of 91 +/- (SE) 2%. However, combined treatment with identical (10 ng/ml) maximally effective doses of both TGFs had little or no effect on the FSH-stimulated accumulation of estrogen, suggesting mutual neutralization by the opposing actions of these peptides. Further evaluation of the antagonistic interaction of the TGFs revealed it to be dose-dependent in that maximally effective doses of TGF alpha (10 ng/ml) partially overcame the stimulation of aromatase activity brought about by relatively low (less than 0.3 ng/ml) but not higher (greater than 1 ng/ml) concentrations of TGF beta, thereby shifting the TGF beta dose-response curve to the right. Treatment with either TGF had no significant effect on granulosa cell DNA content or synthesis, plating efficiency or viability. Taken together, these findings suggest that picomolar concentrations of exogenously provided TGF alpha TGF beta exert potent but diametrically opposed effects on the acquisition of granulosa cell aromatase activity and that the interaction between these two peptides is antagonistic in nature. Our findings further suggest that these direct cytodifferentiative effects of the TGFs may represent intrinsic novel properties of these peptides distinct from their well-established role in the regulation of cellular growth.