Concentrations of progesterone, prolactin, LH and 13,14-dihydro-15-keto-prostaglandin F2(α) (PGFM) were measured in plasma of eight tammar wallabies at 8-hourly intervals during the end of pregnancy and post-partum oestrus initiated by removing the pouch young, and during the end of the oestrous cycle, similarly initiated. In the non-pregnant cycle oestrus occurred 29.7 ± 0.7 ( mean ± SEM) days after initiation of the cycle, was preceded by a slow decline in progesterone concentration from 1.6 nmol/l to less than 0.64 nmol/l and was followed by a preovulatory peak of LH 5.3 ± 3.9 h later. In the pregnant cycle birth occurred 26.1 ± 0.2 days after removing the pouch young and was followed 8.0 ± 2.1 h later by oestrus and 16.0 ± 2.5 h by an LH peak. The latter events thus occurred with a very rapid decline in progesterone and a transient high peak of prolactin. In two females sampled less than 25 min after parturition there was a transient peak of PGFM but in all others the concentrations of PGFM remained basal throughout. It is suggested that the fetus and/or placenta is involved in both the premature decline in progesterone and the initiation of parturition and that onset of oestrus and ovulation, being a consequence of a decline in progesterone, are therefore also determined by the fetus.