Carnitine concentrations in serum, liver, kidney, muscle and heart were determined 30 min, 2 hr and 4 hr after administration of single 50 mg/kg doses of valproic acid (VPA) or octanoic acid (OTA) of fasting mice. Half an hour post-administration (p.a.) of VPA, free carnitine concentrations were smaller than in controls in serum, liver, kidney and heart. Four hr p.a., the effects of VPA had disappeared from all the carnitine sources, which now had concentrations that were not significantly different from those of controls. The effects of OTA are different from, and sometimes the opposite of, those of VPA, showing that the effects of VPA are specific to it. Hyperammonemia, on the other hand, was greatest 4 hr p.a. of VPA. These findings show that the effect of VPA on carnitine metabolism is immediate but transient, and accordingly suggest that the carnitine deficiency observed in patients under prolonged treatment with VPA-containing anticonvulsants must be due to a more complex mechanism than direct interaction between carnitine and VPA.