The effect of age on thyroid hormone-induced lipid peroxidation in rats was determined by measuring ethane exhalation (EE) rate. The mean basal EE rates (in pmol.min-1 x 100 g-1) at room air in 25-mo-old aged rats [3.07 +/- 0.26 (SE)] and in intermediate age (18-mo-old) rats (3.42 +/- 0.18) were significantly lower than that in young (3- to 5-mo-old) rats (4.44 +/- 0.27) (P < 0.01). After 8 days of L-3,5,3'-triiodothyronine (T3) administration, EE rates in aged rats (4.10 +/- 0.25) and in intermediate-age rats (4.16 +/- 0.35) were significantly lower than that in young rats (5.61 +/- 0.36) (P < 0.01). T3-treated young rats pair-fed with aged rats for 2 wk had EE rates (5.53 +/- 0.18) comparable to ad libitum-fed young rats, although the basal rate of EE was significantly reduced with partial food restriction (4.44 +/- 0.27 vs. 3.50 +/- 0.24) (P < 0.01). These observations indicate that reduced food intake with age cannot account for the age-related changes in T3-stimulated peroxidation rate of lipids.