Studies performed on iron-deficient and control rats demonstrated that oxidative energy production (phosphorylation) by mitochondria from iron-deficient red and intermediate skeletal muscles was greatly reduced with pyruvate-malate, succinate, and a-glycerophosphate as substrates. Although phosphorylation was also decreased in iron-deficient white skeletal muscle with succinate and pyruvate-malate as substrates, no change was found with a-glycerophosphate as substrate.