Acetyl-CoA-carboxylase activities were measured in adipose tissues of pigs during a breeding experiment for a low-fat line, and of rats and obese mice under different nutritional conditions. Acetyl-CoA-carboxylase behaves uniformly with the four major NADPH-generating dehydrogenases, like a block of lipogenic enzymes, and is found to be genetically determined in pigs. Correlation with body fat under a variety of experimental conditions confirms the rate-limiting character of acetyl-CoA-carboxylase, not only for the biosynthesis of fatty acids, but obviously also for their esterification and for triglyceride deposition. Activity ratios of this enzyme in different adipose tissues, e.g. outer versus inner layer of subcutaneous adipose tissue in pigs, epididymal versus subcutaneous, or epididymal versus perirenal adipose tissue in rats and obese mice, correlate well with predicted fattening in pigs and with fat deposition in laboratory rodents. Moderate biotin deficiency in obese mice leads to a preferred fat deposition in the epididymal fat pad in comparison with normal biotin supply. The concept of a lipogenic potential in the body is derived from the activity ratios of acetyl-CoA-carboxylase.