The ob gene product leptin is secreted from adipose tissue. Leptin has dramatic effects on food intake and energy expenditure in rodents. Brown adipose tissue is the first form of adipose tissue to appear during development, and is present at birth in most species. The development of a leptin feedback system in early life and the relative role of the brown and white adipose tissues have not yet been revealed. We have investigated the expression of ob/leptin mRNA in brown adipose tissue around birth and with respect to feeding. Northern blotting analysis and in situ hybridization experiments demonstrated the presence of leptin mRNA in brown adipose tissue at 0, 18, and 24 h after birth. The leptin mRNA level was decreased at 8 h postpartum in fed animals and at 18 or 24 h in the absence of feeding. In addition, circulating leptin was detected in the plasma of newborn rats at 0, 10, or 24 h after birth, whereas it was not detectable in 10 h-old animals that did not suckle at their mother. The presence at birth of ob mRNA and circulating leptin, as well as the early effect of suckling on ob mRNA levels, suggests the precocious involvement of leptin in the control of food intake.