The effects of maturation on the inactivation and degradation of cortisol-4-[<sup>14</sup>C] in the lenses of rats were investigated. Inbred Wistar strain rats were used at the 4th and 10th postnatal day for the immature stage and the 60th postnatal day for the mature stage of male rats. Differences were found in the cortisol-binding capacity in the rat lens between the immature and mature stage. No change with age was found for sulphate conjugation, whereas glucuronide conjugation was found to increase with age. The most interesting fact was that degradation of cortisol-4-[<sup>14</sup>C] by lens homogenate is carried out actively in the mature stage, whereas only the side-chain loss metabolite of cortisol-4-[<sup>14</sup>C] is detectable in immature stage. These findings suggest that there is an age difference in the metabolic pattern of cortisol in the rat lens and that this might be closely related to the induction of enzymes involved in the metabolism of cortisol during maturation.