Ovarian theca cells are the predominant source of gonadotropin-stimulated androgen biosynthesis in vivo. Troglitazone (TG), a synthetic agonist of the peroxisome proliferator-activated receptor γ (PPARγ) and a thiazolidinedione used to treat insulin resistance, decreases serum androgen concentrations in women with hyperthecosis and/or polycystic ovary syndrome. Using reverse transcription-polymerase chain reaction (RT-PCR), we demonstrated the presence of PPARγ mRNA in the porcine ovary. Since activation of ovarian PPARγ may alter hormone-stimulated steroidogenesis in vitro, we cultured porcine theca cells for 48 h in the presence of two different PPARγ ligands, TG and 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂). Putative TG-mediated activation of PPARγ resulted in a 53%–69% decrease in LH- and/or insulin-stimulated androstenedione and testosterone accumulation. Although TG reduced 3-isobutylmethylxanthine-enhanced LH-stimulated cAMP accumulation by 74%–78%, it did not alter basal cAMP concentrations. Exposure to 8Br-cAMP did not overcome the TG-induced inhibition of androgen accumulation. In contrast, TG administration amplified basal and hormone-stimulated progesterone accumulation, particularly in the presence of insulin, without altering levels of 17α-hydroxyprogesterone. The putative natural PPARγ ligand, 15d-PGJ₂, inhibited androgen biosynthesis and stimulated progesterone production. RT-PCR-based amplification of cytochrome P450 cholesterol side-chain cleavage (CYP11A) and cytochrome P450 17α-hydroxylase/C-17,20-lyase (CYP17) transcripts indicated that TG moderately enhanced expression of these genes. However, TG did not affect CYP17 protein expression. We conclude that putative ligand-mediated activation of PPARγ decreases LH- and/or insulin-driven theca cell androgen production by impairing the ability of CYP17 to synthesize androstenedione from available progestins. The corresponding augmentation of progesterone production could suggest that PPARγ activation induces theca cell differentiation toward a progestin-synthesizing phenotype.