The role of granulosa cells on porcine follicular androgen synthesis was studied. Granulosa cells were cultured for 24 h either in Eagle's minimum essential medium (MEM) or in MEM plus follicle-stimulating hormone (FSH; 1 microgram/mL). Thecal preparations were cultured with or without luteinizing hormone (LH), either in MEM, 50% "spent media" from granulosa cells control (MA) or 50% "spent media" from granulosa cells incubated with FSH (MB). In the absence of LH, MB stimulated accumulation of both delta 4-androstenedione (twofold) and testosterone (threefold). MB was found to contain high levels of C21-steroids, including progesterone and material which behaves chromatographically and immunologically like pregnenolone. These amounts of C21-steroids were able to stimulate thecal androgen production. The results suggest that granulosa cells contribute to thecal androgen production by providing steroid substrate. This would provide a means for granulosa cells to control the availability of aromatizable androgens.