Administration of nicotinamide to Wistar rats (100 mg/kg body wt.) bearing Yoshida sarcoma (ascites) tumour as well as to Swiss and CBA mice (250 mg/kg body wt.) bearing the transplantable fibrosarcoma and the spontaneously induced mammary carcinoma, respectively, was shown to bring about a reversal of the decreased activity of NADPH-cytochrome c reductase in host livers of the tumour-bearing animals. Nicotinamide injection was also shown to bring about a significant increase in the levels of host hepatic cytochromes P-450 and b5 and in the activities of aminopyrine demethylase and UDP-glucuronosyl transferase which were shown to be low in the tumour-bearing rats. Treatment with nicotinamide was shown to be equally effective in reversing the inhibited activities of hepatic drug metabolising enzymes observed in healthy adult rats injected with serum from the tumour-bearing rats. Administration of nicotinamide to adult male rats brought about an increased incorporation of [14C]leucine in hepatic microsomal proteins which was insensitive to inhibition by actinomycin D at a dose of 1 mg/kg body wt. but could be inhibited by the antibiotic at a lower dose of 0.1 mg/kg body wt. as well as by cycloheximide.