Concentrations of prolactin and progesterone in the plasma of female tammars (Macropus eugenii) were measured during lactational quiescence and seasonal quiescence and during the period of natural resumption of reproduction after the summer solstice in December. Prolactin concentrations were consistently low (less than 40 ng/ml) during the period of declining day length and consistently elevated (greater than 40 ng/ml) during the period of increasing day length. Basal levels of progesterone were lowest (118.9 +/- 9.1 pg/ml) at the winter solstice and highest (244.1 +/- 21.0 pg/ml) at the summer solstice. Treatment with bromocriptine (CB154) did not depress prolactin levels at either time of the year but during February to June a single injection of 5 mg/kg body wt induced development of the quiescent corpus luteum (CL) and the diapausing embryo was reactivated. In February and March the level of progesterone increased in association with the developing CL, but after treatment at the winter solstice in June plasma progesterone did not increase although pregnancy was successfully completed. From September through November only 1 of 50 females responded to bromocriptine. None responded to bromocriptine given at the summer solstice in December, but all these animals spontaneously reactivated 2 weeks later. The difference in response of female tammars to bromocriptine treatment in the two halves of the year suggests that different endocrine controls operate in lactational and seasonal quiescence and that the latter is more complex.