Adult male rats were fed a standard diet containing 25 mg/kg L-thyroxine for 2 weeks. The hyperthyreotic condition of the animals was checked by monitoring the metabolic rates and liver glycerol-3-phosphate dehydrogenase. In the postnuclear fraction of the lung the activity of fatty acyl-CoA oxidase, the enzyme responsible for the rate limiting first step of peroxisomal fatty acid beta-oxidation, showed a twofold increase. Catalase, the marker enzyme of peroxisomes, showed a similar increase. Electron microscopic examination of alveolar type II cells did not reveal changes in the number and distribution frequency of peroxisomes and lamellar bodies. Similarly the content of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (dipalmitoyl phosphatidylcholine), the main constituent of alveolar surfactant, was not altered significantly by thyroxine feeding. On the other hand the volume density of the peroxisomal compartment was found to be doubled according to the measured increase of catalase and acyl-CoA oxidase. Our data suggest that the induction of peroxisomal matrix enzymes, such as catalase and fatty acyl-CoA oxidase, does not influence the surfactant content.