The activities of enzymes of the tricarboxylic acid cycle were measured in order to compare the respiratory capacity in different parts of the nephron of the rat. Oxoglutarate dehydrogenase, citrate synthase and isocitrate dehydrogenase were assayed in single nephron segments dissected out of freeze-dried cryostat sections. The activities of the three enzymes per unit weight are higher in the distal segments (thick ascending limb and distal convoluted tubule) than in the proximal tubule. The distal vs. proximal ratios of activities are about 1.5, 2.5 and 2 for oxoglutarate dehydrogenase, citrate synthase and isocitrate dehydrogenase, respectively. Oxoglutarate dehydrogenase shows the lowest activities along the whole nephron and appears to catalyze the rate-limiting step of the tricarboxylic acid cycle. The possibility to estimate the respiratory capacity in the different segments of the nephron on the basis of the activity of oxoglutarate dehydrogenase is discussed. The capacity calculated for the proximal tubule (between 44 and 66 mumol O X min-1 X g-1, depending on the substrate) is in agreement with direct measurements of oxygen consumption as well as with calculations made on the basis of morphometrical data.