4-Pentenoic acid has been shown to have a hypoglycemic effect on mice in vivo (1) and to inhibit gluconeogenesis and lactate oxidation in pigeon liver homogenates (2). Gluconeogenesis from alanine or pyruvate is inhibited by 4-pentenoic acid in perfused rat liver (3,4), and glucose production from pyruvate and succinate is decreased in rat kidney slices (5). 4-Pentenoic acid also inhibits the oxidation of fatty acids in a variety of systems (1-8). Since fatty acids are known to stimulate glucose production by the liver (9, 10), the impairment of gluconeogenesis has been attributed to a suppression of fatty acid oxidation. It has been suggested that inhibition of fatty acid oxidation is mediated by lack of free CoA and carnitine as a result of the accumulation of non-metabolizable CoA and carnitine esters formed during the oxidation of 4-pentenoic acid (2,6). However, no detailed studies on possible changes of CoA intermediates in tissues have been reported. In the present work, the kinetics of the effects of 4-pentenoic acid on adenine and pyridine nucleotides, CoA and carnitine derivatives have been determined in the perfused rat liver. These studies, and data obtained with intact and sonicated mitochondria show that the metabolic deficiencies observed in liver are caused both by inhibition of β-oxidation, and by depletion of CoA and acetyl-CoA. © 1969.