In the rat, the acetyl-coenzyme A carboxylase gene exists as a single copy per haploid chromosome set. However, multiple forms of acetyl-coenzyme A carboxylase mRNA exist, the relative abundance of which varies in a tissue-specific manner under different physiological conditions. In the mammary gland, the major acetyl-coenzyme A carboxylase mRNA species are of the class 2 type, which are products of promoter II. In parametrial white adipose tissue, the main form of species of acetyl-coenzyme A carboxylase is of the class 1 type, which are produced by promoter I. Pregnancy and lactation affect the amounts of these acetyl-coenzyme A carboxylase mRNA. Although the mammary gland acetyl-coenzyme A carboxylase mRNA species increase dramatically upon parturition, the parametrial white adipose tissue forms decrease precipitously at the same time and are not expressed at all during the lactation period. In the liver of these animals, the only form of acetyl-coenzyme A carboxylase mRNA that is expressed is the FL56 form; this form shows a modest decrease during pregnancy that is slowly reversed during lactation. These observations indicate that the changes in lipogenesis that occur during pregnancy and lactation are determined by the transcriptional activity of the acetyl-coenzyme A carboxylase gene. In order to analyze the complex transcriptional activity of this gene in a meaningful way, it is necessary to examine the metabolism of individual isoforms of acetyl-coenzyme A carboxylase mRNA.