Previous studies suggested that brown adipose tissue (BAT) provides a source of circulating leptin in the newborn mouse. However, we detected no leptin mRNA in newborn BAT or in newborn liver or stomach. In contrast, leptin expression was detected readily in newborn white adipose tissue (WAT). Fasting plasma leptin concentrations in newborn mice were comparable to those in adult nonpregnant mice. In contrast to adult mice, however, the fasting leptin levels in newborn males were comparable to those in females. While leptin in the late-gestational pregnant mouse circulates as a complex with the leptin-binding protein, leptin in the newborn mouse circulates as the free hormone. Factors regulating the expression of leptin by newborn WAT remain to be elucidated.