Total lipid, protein and glycogen concentrations were determined for the diaphragm and interscapular brown adipose tissue of fetal (22-31 days of gestation) and neonatal rabbits. In the diaphragm, protein concentration doubles and lipid concentration drops to half its initial value during the period under study; the glycogen level reaches its maximal value at 27 days, and decreases thereafter. In the brown fat, the lipid concentration rises steadily until the immediate prenatal period; the protein concentration remains steady and the glycogen is highest at 29 days of gestation. Fetal plasma glucose and insulin levels increase steadily; there exists a positive correlation between fetal plasma insulin levels and fetal body weight.