In Exp. 1 non-pregnant female tammars were injected, on Day 26 (the day parturition would normally occur) after removal of pouch young, with saline, 200 micrograms ovine prolactin or 5 mg PG and changes in plasma concentrations of progesterone, prolactin, PGF-2 alpha metabolite (PGFM), oestradiol-17 beta and LH were determined. Luteolysis occurred in females treated with prolactin alone, while treatment with PG first induced a rapid rise in prolactin and subsequently a significant decrease in plasma progesterone. After prolactin treatment the oestradiol peak, oestrus and the LH surge were advanced significantly compared to the saline-treated females. In Exp. 2 the effects of the same treatments as used in Exp. 1 were determined on Day 23 and again on Day 26 after removal of pouch young in non-pregnant females. On Day 23 both prolactin and PG induced significant elevations in plasma progesterone, but luteolysis did not occur. On Day 26 the treatments initially induced significant elevations in plasma progesterone but these were followed by luteolysis within 8-12 h after treatment. PG treatment induced parturient behaviour in the non-pregnant females within 3-21 min and this persisted during the period that plasma concentrations of PGFM were elevated. The results show that PG induces birth behaviour and the release of prolactin, while prolactin first induces an elevation of plasma progesterone concentrations and, in the mature CL on Day 26, subsequently induces luteolysis.