Glycogenin is a self-glycosylating protein required to initiate glycogen biosynthesis. Utilizing the differential display technique to analyze changes in gene expression during early postnatal cardiac development, we have isolated and cloned a 484 bp cDNA fragment that corresponds to the 3' end of rat glycogenin. Northern blot analysis on neonatal cardiac tissues demonstrated hybridization to a 1.7-1.8 kb transcript, which was highly expressed at 3 days and at progressively reduced levels at 1, 2, 3 and 4 weeks of age. A 1624 bp fragment of rat glycogenin was cloned by RT-PCR that includes a 1002 bp open reading frame encoding a 333 amino acid protein. At the nucleotide level, rat glycogenin exhibited 87.2 and 83.6% identity with human and rabbit glycogenin over the open reading frame. The deduced amino acid sequence showed 86.7 and 83.4% identity with human and rabbit sequences, respectively. Given the significance of glycogenin in glycogen biosynthesis, the results of this study suggest a possible molecular basis for the regulation of glycogen during early postnatal cardiac development. In addition, the nucleotide and amino acid sequences of rat glycogenin may be used to investigate the physiological and pathophysiological roles of glycogenin in rat tissues.