The human progesterone receptor (PR) is expressed as two isoforms, PRA and PRB, which function as ligand-activated transcription factors. In-vitro studies suggest that the isoforms differ functionally and that their relative expression in a target cell may determine the nature and magnitude of response to progesterone. We have shown recently that PRA and PRB are co-expressed in target cells of the human endometrium. The purpose of this study was to investigate the homogeneity of expression of PRA and PRB in target cells of the human uterus throughout the menstrual cycle. In the functionalis, PRA and PRB were expressed in comparable levels in glandular epithelium during the proliferative phase of the cycle, whereas there was persistence of PRB but not PRA in the glands during mid-secretory phase. In the stroma, there was predominance of the PRA isoform throughout the cycle. There was remarkable homogeneity in the relative expression of PRA and PRB in adjacent cells within the same tissue compartment, suggesting that the mechanisms regulating relative PR isoform expression are similarly active in these cells. By contrast, heterogeneity between glands was observed under some circumstances in the functionalis of the endometrium, suggesting PR isoform down-regulation by progesterone to be asynchronous. Heterogeneity was also seen between the glands of the basalis and functionalis of the endometrium implying region-specific responses to hormonal stimuli. This study demonstrates adjacent cell homogeneity in the relative expression of PRA and PRB in normal human endometrial tissue and a differential response to ovarian steroid hormones between cell types and between different regions within the same tissue.