The complex nature of insulin-mediated biological responses has made it difficult to interpret such data. Prior studies in our laboratory had characterized the insulin-mediated increases in hepatic malic enzyme activity in normal and diabetic rats (Drake et al., 1983; Drake and Mucenski, 1985). However, since insulin-mediated regulatory processes have been shown to be tissue specific, we decided to examine malic enzyme activity in the epididymal fat pads of normal, diabetic, and insulin-treated normal and diabetic rats. This data revealed that in direct contrast to the hepatic studies, the normal epididymal fat pad contained the low specific activity malic enzyme molecule. Insulin treatment of both normal and diabetic rats resulted in an increase in epididymal fat pad malic enzyme activity due to increases in both enzyme quantity and specific activity. In order to quantitate epididymal fat pad malic enzyme mRNA levels, we isolated a 2.4 kb malic enzyme specific cDNA which was designated pR ME 1. In both normal and diabetic rats, the observed increases in malic enzyme quantity were directly paralleled by increases in malic enzyme mRNA content. The Northern blot data revealed an apparent differential expression of the malic enzyme mRNA doublet between insulin-treated normal and diabetic epididymal fat pads. This study demonstrates that insulin modulates epididymal fat pad and hepatic malic enzyme activity in a tissue-specific manner utilizing a defined subset of insulin-sensitive parameters involving alterations in enzyme specific activity and/or quantity.