Sulfate incorporation was measured in subcultured glandular epithelial cells from guinea-pig endometrium, treated by estradiol alone or in concert with progesterone. Progesterone significantly increased sulfate incorporation in cellular and secreted macromolecules. However, the greatest effect of progesterone was on the size of the intracellular sulfate pool available for sulfation. This effect of progesterone was correlated with the percentage of cells exhibiting progesterone receptors in vitro.