Lysosomal enzyme activities in pancreatic islets of obese hyperglycemic ob/ob mice aged 3 to 6 months were investigated and compared with those of normal lean NMRI mice of the same age. It was observed that the glycogenolytic glucose-producing hydrolase acid amyloglucosidase displayed a fivefold higher activity in the islets of obese mice than in the islets of normal NMRI mice. However, other islet lysosomal enzyme activities measured, such as N-acetyl-beta-D-glucosaminidase and beta-glucuronidase, were of the same magnitude in both obese and lean mice. A starvation period of 24 hours induced a significant depression of islet acid amyloglucosidase activity in obese as well as lean mice, whereas the activities of N-acetyl-beta-D-glucosaminidase and beta-glucuronidase were unaffected. Further, the activities of other types of islet lysosomal enzymes, such as acid phosphatase and cathepsin D, were also measured in obese mice. These activities were not found to be affected by the actual fasting period. A good correlation (r = 0.815; P less than 0.01) was observed between islet acid amyloglucosidase activity and plasma insulin concentrations in obese mice, whereas no such relationship was apparent with regard to other islet lysosomal enzyme activities recorded. Acid amyloglucosidase activity in liver tissue of the obese mouse was about 30 times lower than that of islet tissue. Further, the activity of liver amyloglucosidase was of the same order of magnitude in obese and lean mice. Similarly, other lysosomal enzyme activities in the liver of obese and lean mice were not strikingly different.(ABSTRACT TRUNCATED AT 250 WORDS)