The mouse obese gene product, expressed specifically in adipose tissue, regulates energy balance in mice. Mutation of the obese gene results in marked obesity and type II diabetes as part of a syndrome that resembles morbid obesity in humans. Here we report the cloning and sequencing of rat obese cDNA. Neither alterations of nucleotide sequence in the coding region nor changes of the gene structure were found in two rat strains with obesity, Zucker (fa/fa) and Otsuka Long Evans Tokushima Fatty. The expression level of obese mRNA in adipose tissue of Zucker (fa/fa) rat was found to be about 4 times that in lean littermates, suggesting some mutation or abnormal expression of the receptor for the obese product in obese rats of this strain.