The influence of glucocorticoids on the neuroendocrine system and behavior of lactating rats is not well known. To evaluate the effects of glucocorticoids on the neuroendocrine system and maternal and aggressive behavior, lactating female rats were treated with dexamethasone or vehicle for 2 h before experiments. Blood samples were collected 15 min after the beginning of suckling to evaluate hormonal changes. To evaluate the maternal behavior of lactating rats, eight pups were placed in their home cages on the side opposite the location of the previous nest, and the resulting behavior of the lactating rat was filmed for 30 min. Aggressive behavior was evaluated by placing a male rat (intruder) in the home cage. Dexamethasone treatment reduced oxytocin and prolactin secretion during lactation and reduced pup weight gain. Relative to control treatment, dexamethasone treatment also adversely affected a variety of maternal behaviors; it increased the latency to build a new nest, decreased the number of pups gathered to the nest, increased the latency to retrieve the first pup, and decreased the percentage of time spent in the arched-nursing position. Dexamethasone treatment, compared with control, also reduced aggressive behavior, as evidenced by an increase in the latency to the first attack, a reduction in the number of front and side attacks, and a decrease in lateral threat and biting. Taken together, our results suggest dexamethasone treatment in lactating rats disrupts prolactin and oxytocin secretion, and this is followed by an attenuation of maternal and aggressive behavior.