Cataracts were produced in rat lenses by either feeding a diet containing 50% galactose or by inducing diabetic condition by intravenous injection of streptozotocin. Proteolysis of crystallins, protease activity of calpain II enzyme (EC 220.127.116.11), and presence of calpain molecule (antigen) were determined at four cataract stages--I, cortical vacuoles, II, vacuoles plus hazy cortex, III, nuclear cataract, and IV, mature cataracts. Calpain activity was normal or moderately elevated at early stages of galactose and diabetic cataracts. Later stages III and IV showed proteolysis of lens crystallins, increased proportion of insoluble proteins, loss of calpain enzyme activity and calpain molecule from the soluble fraction, and reduced amounts of calpain associated with insoluble pellet. In galactose cataract, the largest increase in lens calcium were found when proteolysis was present. These results provide evidence for calpain-induced proteolysis of lens crystallins in two in vivo models of sugar cataracts in rodents.