The hypothesis that the effects of dietary sucrose on hepatic lipogenesis could be explained by an effect on mitochondrial respiration was tested. Two groups of male BHE and Wistar rats were fed, from weaning, either a 65% starch or a 65% sucrose diet. Rats were killed at 50-60 days of age by decapitation, and hepatic mitochondria were isolated. Simulated state 3 and state 4 respiration with succinate, pyruvate, or alpha-ketoglutarate as substrates was determined as were the activities of the alpha-glycerophosphate and malate-aspartate shuttles. Mitochondria from Wistar rats had higher ADP:O ratios than BHE rats for all three substrates. Respiratory control ratios with succinate or alpha-ketoglutarate as the substrates were affected by both diet and strain. Respiratory control ratios with pyruvate as a substrate were affected by diet such that sucrose feeding lowered the respiratory control ratio in both BHE and Wistar rats. Shuttle activities were not as markedly affected by strain or diet. We conclude that mitochondrial respiration may be more sensitive to dietary manipulation than mitochondrial shuttle activity.