This study was conducted to identify proteins synthesized and secreted de novo by the guinea pig uterus. Uterine samples were obtained from cycling, late-pregnant as well as ovariectomized and steroid-treated guinea pigs and cultured with either L-[3H]leucine or L-[35S]methionine. Two-dimensional SDS-PAGE of culture medium followed by fluorography was used to determine proteins synthesized and secreted de novo during a 24-h incubation period. Two complexes of estradiol-stimulated proteins (ESP) were detected. Each complex was composed of 5-7 unique proteins with slightly different isoelectric points. The higher molecular-weight complex had a molecular weight of 65,000-60,000 and an isoelectric point range of 5.2-6.1. The lower molecular-weight complex had a molecular weight of 60,000-55,000 and a similar range of isoelectric points. The two complexes of ESP were not observed in medium of explants from animals that received placebos, were late-pregnant, or were treated with progesterone only. Progesterone administered in combination with estradiol enhanced production of both complexes of ESP to similar degrees. Neither complex of ESP was secreted by the explant culture in the presence of tunicamycin, suggesting that the proteins are glycosylated. These findings demonstrate that the uterus of the guinea pig produces two unique complexes of proteins in response to estradiol stimulation, and all results are consistent with the hypothesis that ESP are contained in the carbohydrate-rich secretory granules of endometrial gland cells.