Microdissected pancreatic islets from fresh pancreas of normal and obese hyperglycemic mice have been subjected to micro-enzymatic procedure. Properties of glucose-6-phosphatase have been analysed, in comparison to those in liver and exocrine pancreas. The specific presence of this enzyme has been demonstrated in pancreatic islet of both strains of mice. It was absent in exocrine pancreas Michaelis constant(Km) calculated for glucose-6-phosphate in normal mouse pancreatic islets varied between 0.74 and 0.84 mm.Gulose-6-phosphatase activity of pancreatic islets has been determined in different functional states is related to insulin secretion. Pancreatic islets of obese mice contain fractionally higher glucose-6-phosphatase than those of normal mice. The enzyme activity in mouse pancreatic islets insignificantly decreased after fasting for three days, but did not decrease significantly after tolubtamide treatment and was markedly reduced after alloxan administration. Fasting and alloxan treatment markedly elevated the enzyme activity in both liver and kidney, whereas a slight decrease and significant increase was found in the enzyme activity of liver and kidney respectively after tolbutamide treatment. The present data indicate that glucose-6-phosphatase is present in both normal and obese pancreatic islets. Involvement of this enzyme has been discussed in relation to metabolic regulation of insulin secretion.