This investigation determines the effect of two isocaloric diet regimens on plasma carnitine and urinary carnitine excretion in man. Seven healthy men were served a high-carbohydrate, low-fat (C) or a low-carbohydrate, high-fat (F) diet for 2 wk, ie, one diet regimen for 4 d followed by a 3-d break and concluded with 4 d more on the other diet regimen. The two regimens contained the same amount of carnitine-rich food. Plasma free carnitine rose significantly from the initial value on F diet and was significantly higher from day 3 than C diet. Plasma acyl carnitine increased on both diets. Urinary excretion of carnitine increased only on F diet. Renal clearance of both free and acyl carnitine was significantly greater on F diet than on C diet. Results showed that composition of a diet with constant carnitine content influenced carnitine metabolism in man.