To investigate whether carnosine can inhibit cataract formation and protect Na⁺-K⁺ATPase against inactivation induced by a glucocorticoid. METHODS: Two hundred and twenty clear lenses cultured in vitro were randomly divided into five groups: control group (DMEM), steroid group (DMEM+Dexamethason 10μmol/L), lower concentration carnosine-treated group (DMEM+Dexamethason 10μmol/L+Carnosine 2mmol/L), higher concentration carnosine-treated group (DMEM+Dexamethason 10μmol /L+ Carnosine 5mmol/L) and carnosine group (DMEM + Carnosine 5mmol/L). Progression of cataract formation was evaluated daily using a dissecting microscope. On 1, 3, 5 and 7 days, 10 lenses of every group were homogenized and the activity of Na^{+-K+ATPase was measured by using spectrophotometer. RESULTS: During the incubation, mistlike opacity was observed in the lenses of the control group and carnosine group, but in the steroid group appeared dense nuclear opacity, while both two carnosine-treated groups came out visible demarcation between nuclear and cortical regions on 7 days. A decrease in the activity of Na+-K+ATPase was found in the lens of the steroid group. On 3, 5, 7 days, Na+-K+ATPase activity decreased 22.34% (P =0.002), 47.98% (P <0.001), 75.37%(P <0.001) compared with that at 1 day, respectively. In the carnosine group, the activity of Na+-K+ATPase remained at the level of the control throughout the 7-day incubation, indicating that carnosine itself did not interfere with the original lens enzyme activity. In the lower concentration carnosine-treated group, on 3, 5, 7 days, the activity of Na+-K+ATPase increased 10.8% (P <0.05), 44.6% (P <0.01), 57.4% (P <0.01) of control activity, respectively. In the higher concentration carnosine-treated group, the activity of Na+-K+ ATPase increased 11.3% (P <0.05), 45.7% (P <0.01), 57.6% (P <0.01), respectively on 3, 5, 7 days. The activity of Na+-K+ATPase in both two carnosine-treated groups were only 6.7% and 6.5% lower than that of the control group after 7-day incubation. After the 7-day incubation, the Na+-K+ ATPase activity of the lenses in the steroid group decreased significantly compared with carnosine-treated groups(P ＜0.01). CONCLUSION: Carnosine prevents the cataract formation induced by a glucocorticoid, and significantly inhibits the inactivation of Na+-K+ATPase induced by a glucocorticoid.}