The objectives of the present study were to determine the interaction among basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), insulin, and insulin-like growth factor-I (IGF-I) on cell numbers, steroidogenesis, and IGF-I binding sites in cultured thecal cells. Cells from large (> or = 8 mm) follicles were collected from cattle and cultured for 4 days. Treatment with 1-30 ng/ml of bFGF for 2 days increased (p < 0.05) thecal cell numbers but inhibited (p < 0.05) androstenedione and progesterone production in the presence of insulin alone or insulin plus LH. Treatment with 1-30 ng/ml of bFGF for 2 days also inhibited (p < 0.05) the number of specific 125I-IGF-I binding sites in thecal cells, with maximal inhibition being detected at 1 ng/ml; coculture with 100 ng/ml LH did not influence this effect. In the absence of bFGF and the presence of LH, 30 ng/ml of IGF-I increased thecal cell numbers and production of androstenedione and progesterone. However, cotreatment with 10 ng/ml bFGF inhibited thecal cell response to IGF-I. Cotreatment consisting of 3-100 ng/ml EGF with insulin increased (p < 0.05) thecal cell numbers but decreased (p < 0.05) androstenedione and progesterone production and the number of specific 125I-IGF-I binding sites in thecal cells, with maximal inhibition observed at 3 ng/ml of EGF. Also, cotreatment consisting of 3 and 10 ng/ml of EGF with IGF-I plus LH inhibited (p < 0.05) cell numbers and production of androstenedione and progesterone by thecal cells. These results suggest that bFGF, EGF, insulin, and IGF-I may interact to play a significant role in basal and LH-modulated thecal cell steroidogenesis and mitogenesis during follicular development in cattle.