Rat lenses were cultured in a medium containing either 30mM xylose or 30mM glucose. Inhibitory effects of two aldose reductase inhibitors, AD-5467 and CT-112, on the accumulation of sugar alcohols and the concentrations of the drugs in the lens were measured. Inhibitory effect of lens opacification by the drugs were also observed.Lens opacification was apparently inhibited in a medium containing each of the drugs. Inhibition rates for sugar alcohol accumulation by AD-5467 and CT-112 were unrelated to the kinds of added sugars. 50% inhibition-concentration in the medium was 0.82–1.02×10-7M for AD-5467 and 3.78–5.92×10-7M for CT-112, and the resulting concentration of drug in the lens was 2.33–3.08 nmole/g lens for AD-5467 vs. 21.19–24.63 nmole/g lens for CT-112. The results above indicated that the concentration of CT-112 should be 4.6–5.8 times of that of AD-5467 in the medium to attain the same inhibitory effect on sugar alcohol accumulation, and 8.0–9.1 times more CT-112 is needed in the lens.A linear correlation was found on a bilogarithmic graph of concentrations in the medium and in the lens for both drugs. This correlation is an empirical adsorption isotherm shown by Freundrich. So this suggested that both drugs were adsorbed into the lens. The rate of adsorption varied with the drugs. Even when the drug concentrations in the medium were the same, the concentration of CT-112 in the lens was 2–4 times higher than that of AD-5467.On the other hand, the 50% inhibition-concentrations of AD-5467 and CT-112 on rat lens aldose reductase were 2.58×10-8M and 3.46×10-8M, respectively, suggesting that the two drugs were not very different from each other in aldose reductase inhibitory effect.The above results indicate that a comparison of the dose-effect relation in a target organ is essential for the evaluation of comparative effectiveness of two drugs.