Glycosidases are lysosomal enzymes that participate in the catabolism of glycoproteins and other glycoconjugates, and in some way may modify their activity in situations in which carbohydrate metabolism could be altered, such as the case of obesity. Using a fluorometric assay, a study was made of four glycosidase activities: N-acetyl-β-D-hexosaminidase (NAG), α-mannosidase and α- and β-glucosidase in the serum, pancreas, liver and kidney of 22 Zucker fa/fa genetically obese rats and of 23 fa/? controls, both with ages ranging between 13 and 15 weeks. After 12-14 hours fast and prior anaesthesia with sodium pentobarbital intraperitoneally, blood and the aforementioned organs were removed for enzymatic study of the serum and the organs after homogenization and centrifugation. In the serum a statistically significant increase in α-mannosidase (p < 0.0001) and α-glucosidase (p < 0.02) activities was found in the fa/fa obese rats as compared with the controls. No statistically significant differences were found in serum hexosaminidase activity between the two groups, and no serum β-glucosidase enzymatic activity was detected. In liver, a decrease was observed in hexosaminidase (p < 0.002) and α-glucosidase (p < 0.01) activities in the obese rats as compared with the controls. In whole pancreas an increase was found in α-glucosidase activity in the obese rats with respect to the controls (p < 0.001), with no statistically significant differences in the hexosaminidase, α-mannosidase and β-glucosidase activities. Finally, in the kidney an increase was also observed in α-glucosidase activity (p < 0.01) in the fa/fa obese rats, with no statistically significant differences in the activities of the other three enzymes. In general, the kidney was the organ displaying the highest activities of all four glycosidases studied, both in the obese rats and in the controls. Our findings suggest that changes occur in the activities of some glycosidases in experimental obesity as a result of the metabolic modifications that take place in this situation and that the kidney could play an important role in glycoprotein metabolism.