2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) caused a depletion of serum thyroxine, but paradoxically did not change L-3,5,3'-triiodothyronine (T3) levels in serum of rats. The activities of the thyroid-regulated enzymes alpha-glycerol phosphate dehydrogenase (GPD) and malic enzyme (ME) were determined in livers of normal and thyroidectomized (THX) rats treated with 0.1 to 100 nmol TCDD/kg body weight. Mitochondrial GPD activity did not change significantly as a function of TCDD dose in either normal or THX rats. ME activity was induced by TCDD in a dose-dependent fashion, but only in non-THX animals. The absence of ME induction in THX rats treated with TCDD indicates that TCDD is not intrinsically thyromimetic. The dependence of ME induction on thyroid hormones is much like the thyroid-hormone-dependent, multihormonal induction of ME by insulin and glucocorticoids. However, TCDD had no additive or synergistic effects on induction of ME activity in THX rats fed T3. A 30% decrease in steady-state plasma T3 levels of T3-fed animals treated with TCDD relative to T3-fed controls suggested that T3 catabolism was more rapid in TCDD-treated rats than controls. Thus a thyroid-hormone-dependent, multihormonal interaction is suggested as the basis for induction of ME by TCDD, but a strictly T3-dependent process has not been ruled out.