Intra-arterial administration of 0.25 ml physiological saline to the non-pregnant goat between days 12 and 20 of the oestrous cycle did not affect luteal regression, which was characterized by decreasing peripheral plasma progesterone concentration, beginning on day 13 of the oestrous cycle, and an increase in the plasma concentration of 13, 14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) as oestrus approached on about day 20. Intra-arterial administration of oxytocin antagonist (OA) in saline at a dose of 0.2 microgram kg-1 body weight to goats between days 12 and 20 of the cycle significantly (P less than 0.001) delayed luteal regression beyond day 20 (to day 26). Injection of OA maintained plasma progesterone secretion at 4-5 ng ml-1 till day 23 of the cycle and suppressed the increase in PGFM concentration. Corpus luteum extract (100 microliters) of OA-treated animals released a significant (P less than 0.001) amount of PGF2 alpha from rat uterus in vitro as did authentic oxytocin. This oxytocic material failed to release PGF2 alpha during luteolysis in the goat, suggesting that oxytocin receptors for PGF2 alpha release may be occupied by OA. It is concluded that oxytocin-receptor interaction in the uterus may be the stimulus for PGF2 alpha release which triggers luteal regression in the goat.