Studies show that insulin deficiency enhances peroxisomal enzyme activities. It is not known, however, whether hyperinsulinemia exerts the opposite effect on peroxisomes. Male Sprague-Dawley rats were infused with normal saline, glucose or galactose for 7 days. Only glucose caused an increase in serum insulin levels. The increase in insulin secretion, in response to glucose, was blocked with diazoxide. Data show an inverse relationship between serum insulin levels and hepatic peroxisomal beta-oxidation (r2 = 0.90, p < 0.01). While hyperinsulinemic rats had diminished peroxisomal beta-oxidation, lowering serum insulin restored peroxisomal enzyme activity to normal levels. These effects were independent of blood glucose levels (r2 = 0.35). In addition to decreasing peroxisomal beta-oxidation, hyperinsulinemia was accompanied by accelerated animal mortality, an effect which was also prevented by lowering serum insulin levels. Peroxisomal deficit may be a potentially lethal consequence of hyperinsulinemia.