Pulmonary aldrin epoxidase activity was determined in 2-, 4-, 6-, 8-, 12-, and 16-week-old male rabbits using isolated perfused lung preparations and in vitro incubation techniques. In the perfusion experiments, it was determined that aldrin epoxidase activity increased with age up to 12 weeks of age. Although there was an increase in the weight of lungs with age, this did not appear to be related to the increase in aldrin epoxidation. In vitro experiments carried out using the same substrate to tissue ratio failed to exhibit an age-related increase in the aldrin epoxidase activity. Extrapolated values for aldrin epoxidation from in vitro experiments yielded much higher values than realized from perfusion experiments. Accessibility of the substrate to the enzymatic sites, saturation of the accessible enzyme system, and limiting availability of the cofactor necessary for the oxidative reaction remain as the possible mechanisms for the discrepancy between perfusion and in vitro incubation studies.