Our laboratory is interested in the gonadal growth regulatory properties of inhibins, members of the TGF superfamily. We have previously shown that female mice lacking inhibins (Inha^-/-) develop granulosa cell tumors and that concurrent loss of p27 accelerates tumor development. It has also been shown that the retinoblastoma protein RB regulates the G1 to S phase transition of the cell cycle by controlling the activity of transcription factors and stabilizing the levels of the cell cycle inhibitor P27. Based on these data, we hypothesized that concurrent loss of Rb and inhibins in the ovary will exacerbate tumor formation. To test this hypothesis, we generated an ovarian granulosa cell conditional knockout (cKO) of Rb using the Cre/lox recombination system, in the background of Inha^-/- mice. Inha^-/-/ Rb cKO females show a modest increase in mortality rates compared to Inha^-/- females. Although histologically similar to Inha^-/- ovarian tumors, tumors from Inha^-/-/ Rb cKO females show increased number of mitotic figures and apoptotic rates. Interestingly, P27 levels are decreased in Inha^-/-/ Rb cKO ovarian tumors, likely due to the combined effect of Rb loss and increased Skp2 expression, which targets P27 to the proteosome. We propose that Rb loss may cause cell cycle delay or arrest, followed by apoptosis and that increases in p107 and p130 levels may compensate for Rb loss. These findings confirm the importance of P27 as a cell cycle regulator in granulosa cells and suggest functional compensation between RB-like proteins in ovarian tumorigenesis.