The aim of this study was to immunologically and biologically detect endothelin-1 (ET-1) in rat corpora lutea (CL). Recently, we established a highly sensitive and specific sandwich-enzyme immunoassay (EIA) for ET-1. Using this assay, the presence of ET-1 was investigated in superovulated ovaries, induced with pregnant mare's serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG), and ovaries from pseudopregnant rats, induced by cervical stimulation. A high concentration of immunoreactive endothelin-1 (ir-ET-1) was found in the CL. On reverse phase-high performance liquid chromatography (RP-HPLC) coupled with EIA, ir-ET-1 was exclusively eluted at the same position as synthetic ET-1, indicating that ir-ET-1 is identical to ET-1. The level of ir-ET-1 was significantly (P less than 0.001) higher in the CL 7 days after hCG injection than it was 4 days after hCG injection. On day 7 of pseudopregnancy (PSP), the ir-ET-1 level was also significantly (P less than 0.001) higher than on day 4 of PSP. These results demonstrated that ET-1 is present in a high concentration in the CL, suggesting a new intraovarian peptide which may have a physiological function in the ovary and which may vary in quantity according to the age of the CL.