Bight enzymes, e.g. lactate dehydrogenase, malate dehydrogenase, fructose-dlphosphate aldolase, sorbitol dehydrogenase, gluoose-6-phosphate dehydrogenase, hexoklnase, phosphofructokinase and pyruvate kinase were estimated quantitatively in the rat lens from 37 to 1,211 days of age, by spectro-photometric methods. The activity was expressed as mU/g LWW.All enzymes measured showed declining activities, but LDH, ALD, SDH, G-6-PDH, HK and PFK gave a significant decrease during ageing when plotted semi-logarithmically from 37 to 1,211 days. SDH and G-6-PDH showed a statistically significant difference between the enzymes from the male and the female lenses. The female lens always had a lower activity than the male lens.Of all enzymes the specific activity, expressed as mU/1 mg protein, was calculated. This specific activity appeared to be rather constant during ageing, except for AID, In the female lenses, the specific activity of 7 enzymes was lower than in the male lenses. For ALD the specific activity decreased significantly in the male lens from 5.32 at 37 days to 0.88 at 1,211 days. In the female lens this significant decrease was from 4.97 to 0.81.