Three newly detected dominant cataract mutations (Asc-1, Cat-3(vao), Tcm) were investigated for effects on osmotic alterations in the lenses of heterozygotes. The lens wet weight was reduced in two mutant lines (Cat-3(vao) and Tcm), and the water content in the lenses of the Cat-3(vao) mice was increased. Moreover, in the cataractous lenses from Cat-3(vao)mice, the sodium-potassium-adenosine triphosphatase (Na+ - K+ -ATPase) activity was enhanced and the ATP concentration, correspondingly decreased. The osmotic variations observed in the Cat-3(vao) mutants might have been due to a metabolic response to the yet unknown, primary pathological event. The lenses of the other two mutant lines (Asc-1 and Tcm) revealed no alterations that could be related to osmotic stress. In no mutant line investigated could a decrease in Na+ - K+ - ATPase activity be demonstrated that was similar to the causative factor in the Nakano mutant line. The Cat-3(vao) mice exhibited some similarities to the Philly mutant line.