Dexamethasone treatment induces premature birth in tammar wallabies. Treatment was administered at one of three times between 1200 h on Day 24 and 0930 on Day 25, and birth occurred 22.8 +/- 0.5 h later, significantly earlier than the time of birth for controls, which was 47.7 +/- 2.3 h after treatment. The neonates from treated females were significantly lighter than control neonates (360 +/- 9 vs. 413 +/- 5 mg), and 60% of these died within 12 h of birth, suggesting that premature birth can lead to neonatal mortality. None of the control neonates died. The patterns of secretion of prolactin, prostaglandin F2 alpha-metabolite (PGFM), and progesterone of control and treated animals around the time of birth were similar. A transient pulse of PGFM was coincident with birth while prolactin levels in plasma increased before, and progesterone concentrations fell steeply immediately after, parturition in both groups of animals. The only difference between control and treated animals was in the timing of the hormonal events, which, along with birth, was significantly advanced by the treatment. We conclude that cortisol may play a role in triggering parturition in this marsupial species.