During lactation the efficiency of energy utilization is increased and brown adipose tissue (BAT) thermogenesis is suppressed. We have examined the role of prolactin in the suppression of BAT during lactation in the rat. Changes in BAT were studied using an enzyme-linked immunosorbent assay for the uncoupling protein (UCP). In the first experiment BAT from pregnant and lactating rats was examined at various times. Total UPC content of BAT decreased from 364 +/- 34 to 32 +/- 9 micrograms during late lactation (20 days). Serum prolactin (PRL) concentration was elevated during lactation and was highest at 10 days. When endogenous PRL was stimulated by injection of metoclopramide (1.67 mg/kg) for 5 days, UCP content of BAT decreased by 50%. This action of metoclopramide could be blocked by simultaneous administration of bromocriptine, an inhibitor of PRL secretion. Serum PRL concentration was elevated during metoclopramide administration but not when metoclopramide and bromocriptine were given together. We conclude that PRL is a possible mediator of the suppression of BAT during lactation.