The relationship between the eye lens (the heavier one ef each pair) weight and age in days was studied in 36 known-age Japanese hares, Lepus brachyurus brachyurus, applying the model formula W =a+blog_<10>A (W, lens weight in mg; A, age in days; a and b, coefficients). The results obtained are summarized as follows: 1. It was confirmed that our data for the eye lens weight and age in days fitted the above formula well, and a predicted age in days (Â) for a given lens weight (W) could be calculated from the equation Â=10^<(W+106.632)/117.171>. 2. For instance, a hare with the lens weight of 180.5 mg is estimated at 282 days in predicted age. The 95% confidence intervals (the lower age limit-the upper age limit) of a predicted age of 282 days in the inverse prediction are 42 days (262 days-304 days, 14.9% of the predicted age) for a mean prediction and 265 days (179 days-444 days, 93.9% of the predicted age) for an individual prediction. 3. The age structures of two local populations (Fukuoka and Kagoshima Prefectures) of the Japanese hare were examined using the above equation. As the result, it was found that hares estimated at more than 1 year old occupied 78.2% of 110 hares in the Fukuoka population and 61.1% of 185 hares in the Kagoshima one. In addition, ha res over four years old were observed in the Fukuoka population (29.1%) and Kagoshima population (8.1%).