Different age determination methods were compared to find a practical method for distinguishing between young and adult mountain hares Lepus timidus. The age of the hares was determined using: 1) the ossification stage of the radius and ulna, and 2) the dry weight of the eye lens. The ossification stage of the bones was examined with the unaided eye and palpation, and by using radiography. The proportion of young was higher when the bones were examined using radiography rather than the unaided eye, except in early autumn when the two methods led to the same results. Age determination based on bone examination using the unaided eye gave more reliable results than age determination based on radiography; this became evident when the results were compared with the reproductive status of females or with the dry weight of the eye lens. Using an eye lens weight of 0.25 g as a discriminating value between young and adult hares, gave an age determination similar to that derived from bone examinations using the unaided eye in 90% of the cases. Age determination based on examination of the radius and ulna using the unaided eye and palpation, together with the eye lens weight thus forms a reliable and practical method for distinguishing between young and adult mountain hares, and radiography is only necessary when examining live animals.