The adult germfree rat has a low metabolic rate, and the activity of various hepatic enzymes differs substantially from values found in the conventional animal. To promote understanding of the shifts in intermediary metabolism apparent in the germfree rodent, parameters of thyroid function and the activity of three enzyme systems presumably influenced by thyroid function were determined in 40-, 60-, and 100-day-old male germfree and conventional Lobund rats of Wistar origin. Serum thyroxine concentrations generally increased with age. They were below normal levels in 40- and 60-day-old germfree rats but appeared normal in the 100-day-old germfree animal. Serum thyroxine-binding protein saturation levels were significantly reduced in germfree rats of all age groups, but the reduction was least in the 100-day-old group. Mitochondrial succinate oxidase and alpha-glycerophosphate dehydrogenase activities were lower in livers of germfree rats of all age groups, although the significance of the difference in succinic oxidase activity became less in older animals. Monoamine oxidase activity was similar in germfree and conventional animals. Although the data suggest a hypofunction of the thyroid in the immature germfree rat, no definite relation between thyroid function and the characteristic metabolic anomalies of the germfree rat could be established.