Ovarian development, follicular growth and atresia require mechanisms regulating proliferation and death of ovarian cells including theca-interstitial (T-I) cells. Transforming growth factors-alpha and -beta (TGF-alpha and TGF-beta) are well recognized local modulators of T-I function. This study was performed to evaluate the effects of TGF-alpha and TGF-beta on ovarian T-I cell proliferation, differentiation and apoptosis. T-I cells from immature Sprague-Dawley rats were purified and incubated in chemically defined media. Proliferation was assessed by [3H]thymidine incorporation assay and by cell counting. Steroidogenically active cells were identified histochemically by detection of 3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity. DNA was extracted and apoptosis was identified by detection of internucleosomal DNA cleavage producing the characteristic 'ladder pattern' of low-molecular weight (LMW) DNA following agarose gel electrophoresis. Quantification of apoptosis was carried out with the aid of 3'-end labeling of DNA fragments with [32P]-dideoxy-ATP. TGF-alpha and TGF-beta stimulated [3H]thymidine incorporation by 2.2- to 3.1-fold and 1.7- to 3.4-fold respectively (P<0.005). A combination of TGF-alpha and TGF-beta produced a synergistic increase in DNA synthesis by 6.7-fold (at 1 ng/ml of each TGF-alpha and TGF-beta; P<0.001) and tenfold (at 10 ng/ml of each TGF-alpha and TGF-beta; P<0.001). Cell counting revealed that TGF-alpha increased the total number of cells 2.8-fold and TGF-beta 2.8-fold. The combination of TGF-alpha and TGF-beta increased the total cell count 3.2-fold, compared with control (P<0.05). The percentage of the steroidogenically active cells was 37+/-9% (mean+/-s.e.m. ) in the control cultures, 50+/-5% in the presence of TGF-alpha, 42+/-8% in the presence of TGF-beta, and 47+/-13% in the presence of both TGF-alpha and TGF-beta. TGF-alpha decreased apoptosis by 63+/-14% (P=0.02) while TGF-beta had no statistically significant effect. TGF-alpha in combination with TGF-beta produced the greatest inhibition of apoptosis by 73+/-8% (P=0.01). These findings demonstrate that TGF-alpha and -beta stimulate proliferation of both steroidogenically active and inactive T-I cells. Furthermore, TGF-alpha alone and in combination with TGF-beta protects T-I cells from apoptotic death. These effects of TGFs may be important in physiologic maintenance of ovarian mesenchymal growth and homeostasis as well as in pathophysiologic conditions associated with excessive growth of the T-I compartment, such as polycystic ovary syndrome.