Tryptophan pyrrolase activity has been assayed in crude liver homogenates prepared from rats submitted to chronic ethanol treatment. The results show that chronic ethanol treatment causes a marked reduction in the apoenzyme form of tryptophan pyrrolase. This reduction can be reversed by administration pyridoxine hydrochloride (100 mg/kg i.p. pro die for 8 days). Since chronic ethanol treatment increases cellular levels of NADH and NADPH which are known to inhibit tryptophan pyrrolase activity, it is suggested that the potentiating effect of pyridoxine might be due to the maintenance of normal ratios of cellular NAD+/NADH and BADP+/NADPH. In ethanol treated rats, pyridoxine was as effective as fructose, a regenerator of NAD+ and NADP+ in maintaining control levels of the apo-[tryptophan pyrrolase].