Glucose-6-phosphatase (G6Pase) plays a major role in gluconeogenic pathway. To analyze its regulation, we have cloned a full-length cDNA for G6Pase catalytic subunit from the rat liver. In the cloned cDNA, a 492 base insertion, respective to the previously reported sequence, was found in the 3'-noncoding region. In both ends of this insertion, 5'-and 3'2'splice site motifs were identified. However, spliced mRNA was hardly observed in the rat RNA. Abundant expression was observed in the liver and the kidney by Northern analysis. Expression was also observed in the spleen, adrenal gland and small intestines. Reverse-transcription (RT) polymerase chain reaction (PCR) analysis revealed that G6Pase mRNA was also expressed in a variety of tissues including pancreatic islets. Its expression was increased in the ketotic diabetic mice livers and was corrected by insulin treatment. However, no appreciable changes were observed in kidneys. Broad tissue distribution in expression and the tissue specificity in regulation were thus considered to be the important features of G6Pase gene.