Mid-luteal phase (control) and mid-luteal phase luteal cell suspensions from rats given prostaglandin F2 alpha (PGF2 alpha) to induce luteal regression and killed 24 h later (P24-regressed) were incubated at 37 degrees C in serum-free medium. Progesterone secretion and lipid peroxidation induced by treatment with human chorionic gonadotrophin (hCG) alone or hCG plus phospholipase A2 and PGF2 alpha were measured. Differences were demonstrated in progesterone secretion and lipid peroxide measurements between control and P24-regressed cell suspensions. The experiments suggested that malondialdehyde formation may be a useful indicator of luteal regression and cell death.