The addition of 4-pentenoic acid to liver homogenates results in a decrease in long chain fatty acids oxidation and in gluconeogenesis. Glycolysis is not affected, whereas lactate oxidation to CO₂ is inhibited. There is an increased formation of alanine and decreased levels of glutamate and aspartate. The inhibition of gluconeogenesis is partially reversed by the addition of short chain fatty acids or of (-)-palmitylcarnitine and totally reversed by addition of coenzyme A and carnitine, but not by either one alone. Only the combination of CoA and carnitine partially reversed the inhibition of lactate oxidation. The formation of pentenoylcarnitine and acrylylcarnitine upon incubation with 4-pentenoate has been shown. The data presented suggest that the metabolic effects produced by 4-pentenoate are caused at least partially by accumulation of nonmetabolizable acyl-CoAs and acylcarnitines derived from the acid, thus making both carnitine and CoA unavailable for normal metabolic function.