Velocity sedimentation at unit gravity was used to fractionate collagenase-dispersed deciduoma cells of rats so that the effect of progesterone on the synthetic activity of specific populations of deciduomal cells could be examined. Separation of cells by size using velocity sedimentation yielded cell fractions with 2n–8n content of DNA/cell reflecting the endomitosis characteristic of stromal cell differentiation. Smaller cells containing 2n–4n DNA synthesized DNA early in decidualization whereas the DNA synthesis of cells containing 4n–6n DNA predominated on Day 8 of pseudopregnancy. A large fraction of deciduomal cells which synthesized DNA during the initial stages of decidualization did not then proceed to synthesize DNA later with the development of polyploidy. Progesterone maintained rates of DNA and RNA synthesis in polyploid deciduomal cells containing 4n–8n DNA. Rates of protein synthesis, however, were maintained by progesterone in smaller cells containing 2n–4n DNA. Increased rates of synthesis of specific deciduomal proteins in response to progesterone treatment were not detected by SDS-polyacrylamide gel electrophoresis. These studies indicate that progesterone maintains the synthetic activity of specific populations of deciduomal cells during decidualization.