Radioactive carnitine, in the form of L-[methyl-3H]carnitine, was administered intravenously to male rats and the specific radioactivity of carnitine in blood plasma and 13 tissues was measured for 16 days. There was no evidence of metabolism of carnitine to other compounds. A compartmental analysis was made by comparing the variation with time of the specific radioactivity of each tissue with one of two models. Kidney, heart and epididymal fat were best represented as containing a single compartment of carnitine, whereas spleen, liver, lung, adrenal, prostate, seminal vesicle, pancreas, muscle, testis and brain were best represented in terms of two compartments each exchanging carnitine with blood plasma. Estimates were obtained of the turnover times of carnitine in the individual tissue compartments as well as the fluxes across each compartment boundary. Analysis of the variation in the specific radioactivity of carnitine in urine and blood plasma. Estimates were obtained of the turnover times of carnitine in the individual tissue compartments as well as the fluxes across each compartment boundary. Analysis of the variation in the specific radioactivity of carnitine in urine and blood plasma indicated an average total excretion rate of carnitine of 10.4mumol/day, of which about 3.2mumol was found in the urine.