Factors associated with the development of obesity were compared among obese (fa/fa), heterozygous (Fa/fa) lean and homozygous (Fa/Fa) lean Zucker rats at 17 d of age. Inguinal pad weight, pad-to-body weight ratio and fat cell size were highest in obese pups (fa/fa > Fa/fa > Fa/Fa). Hepatic glucose-6-phosphate dehydrogenase activity was greater in fa/fa than in Fa/Fa pups; 6-phosphogluconate dehydrogenase activity was higher in fa/fa and Fa/fa pups than in Fa/Fa pups, and fatty acid synthetase was greater in fa/fa compared with lean pups (Fa/fa = Fa/Fa). The fa/fa pups had greater adipose tissue glucose-6-phosphate dehydrogenase, malic enzyme and fatty acid synthetase activities than lean pups, which did not differ from one another (Fa/fa = Fa/Fa), whereas 6-phosphogluconate dehydrogenase and lipoprotein lipase activities were highest in obese pups, intermediate in heterozygotes and lowest in homozygous lean rats (fa/fa > Fa/fa > Fa/Fa). Glucose conversion to carbon dioxide and fatty acids in isolated adipocytes was highest in obese pups (fa/fa > Fa/fa > Fa/Fa). Glyceride-glycerol production was greater in Fa/Fa than in fa/fa or Fa/fa pups. These findings indicate that many characteristics of obesity are evident in preobese Zucker rats, and for some factors the presence of the 'fa' gene in lean rats results in intermediate measurements relative to the two homozygous genotypes.