At parturition in the tammar wallaby, Macropus eugenii, the process of luteolysis involves both prostaglandin and prolactin. Prolactin alone is luteolytic, but it has been unclear whether prostaglandin F2 alpha (PGF2 alpha) is also directly luteolytic. To examine this, three groups non-pregnant animals were studied on Day 26 after removal of pouch young (the day equivalent to parturition). Group 1 (controls) received saline injections on Days 25 and 26, Group 2 received saline on Day 25 and PGF2 alpha on Day 26, and Group 3 was pre-treated with bromocriptine (Parlodel LA, Sandoz) on Day 25 and received PGF2 alpha on Day 26. For 12 h after treatment on Day 26, plasma progesterone and prolactin concentrations were determined and behaviour was recorded. Plasma progesterone concentrations remained elevated and plasma prolactin was basal throughout the sampling period in the control animals (Group 1). After treatment with PGF2 alpha (Group 2), plasma prolactin was elevated within 15 min and plasma progesterone decreased significantly (P less than 0.001) from 600 pg mL-1 (1.91 nmol L-1) to less than 200 pg mL-1 (0.64 nmol L-1) by 8 h. However, luteolysis did not occur in females pre-treated with bromocriptine on Day 25 and injected with PGF2 alpha on Day 26 (Group 3), and prolactin concentrations remained basal. All females treated with PGF2 alpha showed parturient behavior. The results clearly show that PGF2 alpha is not directly luteolytic on Day 26 of the non-pregnant cycle, but they confirm that PGF2 alpha induces the release of prolactin, which is directly luteolytic.