Previous studies from our laboratory have demonstrated that postischemic infusion of thyroxin (T4) will augment the restoration of cellular ATP and enhance the recovery of renal function. It has not been clear, however, whether T4 has a direct effect on mitochondrial ATP synthesis or an indirect effect by stabilization of the plasma membrane. To differentiate these putative effects, rats were subjected to 45 min of renal ischemia and given either normal saline (0.5 mL) or T4 (20 μg/100 g body weight) during the first 15 min of reflow. Cellular ATP levels were assessed by 31P-nuclear magnetic resonance spectroscopy, and release of lactate dehydrogenase (LDH) was used as an index of plasma membrane integrity at 30 and 120 min of reflow. In rats given normal saline, renal ATP had returned to only 57.9 ± 1.4% of preischemic values at 30 min of reflow and 66.1 ± 1.4% by 120 min. LDH release was 13 ± 0.89% at 30 min and 14.6 ± 1.6% at 120 min. In contrast, T4-treated animals had ATP levels of 70.2 ± 2.0% at 30 min and 84.0 ± 1.9% at 120 min, whereas LDH release was elevated to values similar to those in normal saline-treated rats, 14.9 ± 1.5% and 14.4 ± 0.5% at 30 min and 120 min, respectively (nonischemic LDH 8.8 ± 0.8%). These data suggest that T4 stimulates the recovery of renal ATP by a direct effect on synthesis rather than an indirect effect related to global improvement in cellular integrity.