Summary. In tammar wallabies kept in normal daylengths between the equinoxes, births occurred at the normal time after the summer solstice and the progesterone profile reflected this cycle. There were no significant changes in plasma prolactin. In tammars placed in a 15L : 9D photoperiod and then, 5 weeks later, in 12L :12D, births occurred 29\p=n-\36 days after the daylength was reduced. A characteristic transient peak of plasma progesterone occurred 8\p=n-\14 days after the change, indicating a delay of 3\p=n-\9 days in the resumption of luteal activity. Plasma prolactin levels were significantly higher (> 40 ng/ml) (P < 0\m=.\01)during the solstitial daylength, declining to <30 ng/ml within 14 days of the reduction in photoperiod. Animals kept in 12L : 12D and then in 9L :15D gave birth asynchronously after entry into the pens, suggesting that this daylength (12L:12D) is permissive. Changes in plasma progesterone reflected the resumption of luteal activity but no significant changes in plasma prolactin were observed.This study shows that the inhibition of the corpus luteum in seasonal quiescence can be maintained in the presence or absence of elevated prolactin levels, indicating that other factors, perhaps of pineal origin, may also be involved.