The purpose of these experiments was to describe the expression of mRNA for calpain II proteolytic enzyme (EC 3.4.22.17) during normal maturation of rat lens and in cataract formation. Quantitative RT-PCR indicated that the concentration of mRNA for calpain II in whole lens was 3-24 times higher than in age-matched rat liver, kidney, lung and brain, and it was at least five times higher than in young human lens. mRNA levels for calpain II were highest in the outer regions of young rat lens at 5 x 10(6) copies microgram-1 total RNA. Early-stage experimental cataract caused increased calpain II mRNA, while mature nuclear cataract showed a 64% loss. In contrast, mRNA levels for GAPDH, beta-actin, and lens-specific structural protein beta A4 remained constant during experimental cataract formation. Unlike the lower and constant levels in rat liver, kidney and lung; calpain II mRNA levels in whole rat lens decreased with age. These data help explain the high enzymatic activity of calpain II in young rat lens, susceptibility of young rat lens to a variety of cataracts showing increased calcium and calpain-induced proteolysis, and low calpain enzyme activity in human lens. Since the up-regulation of calpain II mRNA was more dynamic than either the amounts of calpain II enzyme or proteolysis of crystallins in cortex, resulting proteolytic activity against the bulk of lens proteins seems to be regulated by post-translational factors, such as increased calcium. The precise role of the up-regulation of calpain II mRNA is unknown, but we hypothesize that it may be associated with the initial cataractogenic response in the epithelial cells or peripheral cortical fibers.