Branched-chain alpha-keto and amino acid (BCKA, BCAA) concentrations were measured in blood, plasma, and tissues of rats fed low protein (8% casein) or high protein (60% casein) diets; and in rats fed a stock diet and subjected to 3 days of starvation of chemically-induced diabetes. Concentrations of these amino and ketoacids were also measured in blood from patients with maple syrup urine disease. Valine, isoleucine, and leucine concentrations in blood from rats fed the stock diet were 124 +/- 7, 58 +/- 4 and 99 +/- 5 microM, respectively. Blood BCAA concentrations of rats fed the high protein diet and diabetic rats were elevated 2- to 3-fold; small increases were observed in blood from starved rats. Changes in blood BCAA concentrations paralleled those in tissues, except in starved rats in which the skeletal muscle free BCAA pool increased proportionately more than the circulating pool. Mean blood BCKA concentrations of rats fed the stock diet were low--7.9 +/- 0.5, 7.1 +/- 0.4 and 12.4 +/- 0.7 microM for alpha-ketoisovaleric, alpha-keto-beta-methylvaleric, and alpha ketoisocaproic acids, respectively. All treatments resulted in increases in blood BCKA concentrations of from 1.4 to 2 fold. In liver and heart, concentrations of BCKA, except for that of alpha-ketoisocaproic acid were near the limits of detection (less than 1 nmole/g). There was significant accumulation of all three BCKA in skeletal muscle which was estimated to contain about 80% of the measured body free BCKA pool. Blood BCKA are well regulated. Only in patients with maple syrup urine disease are plasma concentrations of BCKA useful indicators of altered tissue BCAA metabolism. Skeletal muscle, where oxidation of the BCKA is limited by low BCKA dehydrogenase activity, would seem to be the major source of circulating BCKA.