N-Acetyl-L-ethionine sulfoxide, the main urinary metabolic product of L-ethionine in rats, is produced and excreted by hepatocytes, as shown by its identification in liver extracts of rats administered L-ethionine and in the cultivation medium from primary culture of rat hepatocytes incubated with L-[ethyl-1-14C] ethionine. The metabolic fate of N-acetyl-L[ethyl-1-14C] ethionine isolated from the urine of rats injected with L-ethionine was examined in rats in vivo. This compound can be metabolized, to a small extent, into the usual metabolic products of L-ethionine: S-adenosylethionine and, to an increased extent to CO2.