We demonstrate a delaying effect of deep-sea drinking water (DDW) containing enhanced magnesium ion (Mg(2 +)) and calcium ion (Ca(2 +)) concentrations on cataract development in Shumiya cataract rats (SCRs). The lenses of SCRs administered DDW-200 (Mg(2 +), 200 mg/L, Ca(2 +); 71 mg/L) were less opaque than those in SCRs administered purified water (PW). In SCRs administered DDW-200, a decrease in Mg(2 +) content and an increase in Ca(2 +) content in the lenses was prevented in comparison with SCRs administered PW. Nitric oxide (NO) levels in the lenses of SCRs administered DDW-200 were lower than those in SCRs administered PW. In conclusion, the administration of DDW-200 to SCRs has a potent delaying effect on cataract development, possibly due to preventing an increase in NO levels in the lens.