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Immunoreactive endothelin-1 (IR-ET-1) was detected in the cultured medium from endometrial but not myometrial cells of rabbits in primary culture using a specific radioimmuno assay (RIA). Similar results were obtained with a radioreceptor assay using myometrial membranes. In a reverse-phase HPLC synthetic ET-1 and IR-ET-1 of the extract medium from endometrial cells revealed essentially the same elution profiles, as determined by RIA. Two selective agonists of oxytocin (OT) or V1 vasopressin (VP) receptors produced, respectively, a 6- and 2-fold increase of IR-ET-1 release from endometrial cells. These effects were completely reversed by the addition of two specific antagonists of OT and V1 VP receptors. Our results indicate that ET-1 is produced and released in the culture medium of rabbit endometrial cells in primary culture. The release of ET-1 is under receptor-specific control by neurohypophyseal hormones.