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.