Over the first two weeks postpartum there is a decline in the amount of time that the rat dam spends with her young, resulting from a decrease in the duration of each nest bout. The duration of each nest bout is limited by the rate of rise of maternal temperature when she huddles with her litter. This pattern of mother-young contact is dependent on the dam's hormonal status, because adrenalectomised dams fail to show the expected decline in mother-young contact over time. Ovariectomy, on the other hand, does not have any effect on this behavior. Replacement therapy with glucocorticoids or placing the dam in a warm (25 degrees C) ambience reinstates the normal pattern of mother-litter contact in adrenalectomised-ovariectomised dams. These data suggest that the elevated level of serum glucocorticoids in lactating dams affects maternal behavior by increasing maternal heat load, thereby making the dam vulnerable to an acute rise in temperature when huddling with her young. Prolactin suppression also results in an increase in contact time between mother and young, but only in the second week postpartum. The effects of prolactin suppression are reversed by progesterone replacement or placing prolactin-suppressed females in a warm ambience. However, progesterone is ineffective in restoring the normal pattern of mother-litter contact in adrenalectomised females. These findings suggest that progesterone raises the thermal set point, thereby permitting the thermogenic effects of glucocorticoids to occur.