Acute administration of nicotine hydrogen (+)-tartrate enhances the activity of rat liver tryptophan pyrrolase by a hormonal mechanism. Chronic nicotine treatment inhibits, and subsequent withdrawal enhances, the pyrrolase activity. The inhibition during chronic treatment is not due to a defective apoenzyme synthesis nor a decreased cofactor availability. Regeneration of liver NADP+ in vitro and in vivo reverses the inhibition. Chronic nicotine administration increases the liver NADPH concentration. The above effects of nicotine resemble to a remarkable degree those previously shown for morphine, phenobarbitone and ethanol. All effects are compared, and their possible significance in relation to drug dependence is discussed.