The purpose of this experiment was to assess the roles of free, intracellular calcium and calcium-dependent neutral protease (calpain II, EC.34.22.17) in selenite nuclear cataract. Free calcium ion concentrations within lens nuclear fibers during selenite cataractogenesis increased to 3 microM on day 2 post-injection (clear lens) and to 108 microM at day 4 (nuclear cataract). Calpain II is known to be activated in vitro by calcium levels above 50 microM. Calpain II activity was present in the lens nucleus at time periods preceding formation of selenite cataract. These data suggested that after selenite injection, calpain II was activated by elevated free calcium in the nucleus, and that calpain II-induced proteolysis of nuclear proteins was an important mechanism in selenite cataract. Calpain II levels were also observed to decrease in the nucleus during selenite cataractogenesis, probably due to autolysis. This was supported by the finding that incubation of purified lens calpain II with 100 microM calcium caused partial inactivation of the protease.