A cytochemical analysis reveals the development of fatty yolk (FY) in the early antral oocyte and proteid yolk (PY) in the late antral oocyte of the guinea pig. While the FY develops from the lipid elements of the oocyte (endogenous), the PY apparently develops from the protein-positive precursor granules infiltrating into the oocyte (exogenous). Cytochemically the FY is composed of saturated triglycerides and the PY of tyrosine-, tryptophan-, histidine-, arginine-, and -SH and -NH2 groups-containing protein. It has been found possible to conclude that the FY is used up by the growing oocyte, while the PY continues to be present in the mature pre-ovulatory oocyte.