Bone Morphogenetic Proteins (BMP) and their receptors (BMPR) are now known to have important roles in mammalian ovarian folliculogenesis. This study determined the expression of the mRNA encoding for BMPs and their receptors in the chicken ovary and explored possible roles for them. The expression of the mRNA for BMP-2, -4, -6, -7 and BMPR-IA, IB, and II was determined and quantified by a semi-quantitative RT-PCR. The mRNAs for all the BMPs and receptors determined were present in both the granulosa (G) and theca (T) cells of the F1, F2 and F3 follicles. All BMP mRNAs increased in granulosa cells with follicular development whereas only BMP-7 mRNA has this trend in the theca. BMP-2, -4 and -6 mRNAs in theca were similar between follicles. BMPR-IA mRNA was similar in F2G and F3G but lower in F1G. BMPR-IB mRNA was similar in granulosa of all follicles and BMPR-II mRNA increased with development. In the theca, each receptor subtype showed equal distribution between follicles. mRNA levels for BMPR-IB and -II were higher in granulosa than in theca suggesting that the granulosa is a major target for BMPs. BMP-4 and -7 stimulated basal, IGF-I-, and gonadotrophin-stimulated progesterone production by cultured granulosa cells with differential responses between cells from the F1 and F3/4. This suggests involvement in follicular differentiation. BMP-4 and -7 reversed the inhibitory effects of TGF on basal and gonadotropin-stimulated granulosa cell progesterone production with greater effect in the F1 compared to the F3/4. This effect suggests an important role for BMPs interacting with TGF in modulating the effects of gonadotrophins and IGF-I on follicular differentiation. Finally, BMP-7 stimulated granulosa cell proliferation but BMP-4 inhibited TGF + IGF-I or + FSH- stimulated granulosa cell proliferation suggesting a role in the control of follicular growth during development. These effects of BMP-4 and -7 on the granulosa cell function showed relationships with the expression levels of the BMPs and the BMPR-II.