Cataractous lenses have an altered distribution of the intracellular ionic environment, and the lens ionic imbalance with increased levels of calcium (Ca²⁺) and sodium (Na⁺), coupled with decreased levels of magnesium (Mg²⁺) and potassium (K⁺), is related to cataract development in human senile cataracts. We previously found that the decrease of ATP in lenses caused lens ionic imbalance, and probably decrease in ATPase function. In this study, we investigated the effect of Mg²⁺ deficiency on cataract progression using human lens epithelial (HLE) cells. Expression levels of inducible nitric oxide synthase (iNOS) mRNA in HLE cells were significantly greater in Mg²⁺-deficient medium (Mg²⁺ 0.021 mM) than in normal Mg²⁺ medium (Mg²⁺ 0.77 mM). The NO release from the HLE cells cultured with Mg²⁺-deficient medium also increased. On the other hand, the ATP levels in HLE cells 24 h after incubation with Mg²⁺-deficient medium were lower than that with normal Mg²⁺ medium. The Ca²⁺- and Na⁺/K⁺-ATPase activities in HLE cells until 24 h incubation with normal Mg²⁺ or Mg²⁺-deficient medium did not change. Both diethyldithiocarbamate 10 μM and aminoguanidine 250 μM attenuated the increase of NO release, and caused an increase in ATP levels in HLE cells 24 h after incubation with Mg²⁺-deficient medium. These results suggest that Mg²⁺ deficiency enhances NO production via iNOS in the lens. It is possible that the excessive production of NO cause the decrease of ATP levels. These results show that Mg²⁺ deficiency in the lens may cause an acceleration of the progression of lens opacification.