This study determined the presence of specific endothelin isoforms in human endometrium using high performance liquid chromatography (HPLC) combined with radioimmunoassay, and immunocytochemistry to detect the endothelin precursors (proendothelin-1, proendothelin-2 and proendothelin-3). Endothelin-like immunoreactivity was measured in HPLC eluates using antisera raised in rabbits against the carboxy-terminal heptapeptide of endothelin-1, which is common to the three endothelin isoforms. Of eight endometrial samples analysed by HPLC, three were in the proliferative phase of the cycle, and five in the secretory phase. Endothelin-1 was detected in seven samples, whereas endothelin-2 and endothelin-3 were seen in four and five specimens, respectively. No relationship was seen between endothelin isoforms and the stage of the cycle. Immunocytochemistry was performed on five proliferative and three secretory phase tissues. When present, staining for the precursor proendothelins was localized to endometrial glandular and luminal epithelium (proendothelin-1, 5 of 8; proendothelin-2, 5 of 8; proendothelin-3, 6 of 8). In two sections, staining was also seen in vascular endothelium using antibody raised against proendothelin-1 (n = 1) and proendothelin-3 (n = 1). These data provide evidence that the three endothelin isoforms are present in human endometrium, and suggest that these potent vasoactive agents may play a role in the paracrine control of the uterine vascular bed.