Recent studies have suggested that growth hormone-releasing factor (GRF), like vasoactive intestinal peptide (VIP), may enhance follicle-stimulating hormone (FSH)-stimulated steroidogenesis in cultured rat granulosa cells (GC). Because effects of GRF or VIP on GC proliferation have not been reported, we evaluated and compared the effect of GRF to that of VIP using cultured bovine GC. Undifferentiated GC from 1-5 mm bovine follicles were established for 2 days in medium containing 10% fetal calf serum, washed and then cultured in chemically defined medium for an additional 2 days. Two-day treatment with 2.5-1000 ng/ml of VIP had no effect (P greater than 0.05) on proliferation or progesterone production of bovine GC in the presence or absence of 200 ng/ml FSH. In comparison, 100, 250, 500, 1000 or 2000 pg/ml of human [desNH2Tyr1,D-Ala2,Ala15]-GRF(1-29)-NH2 analog caused a dose-dependent stimulation (P less than 0.05) of GC proliferation in the absence and presence of 5 micrograms/ml insulin. However, the GRF analog had no effect (P greater than 0.05) on GC progesterone production (expressed as ng/10(5) cells/24 h) in the absence or presence of 5 micrograms/ml insulin. The effects of GRF analog on progesterone production and cell proliferation were not influenced by co-culture with 200 ng/ml FSH. GRF(1-44)-NH2 also stimulated cell proliferation but had no effect on basal or FSH-induced progesterone production. These results suggest that GRF may play a role in GC proliferation during follicular development in the bovine.