The starting point of this study is the observation that succinate, the well-known Krebs cycle intermediate, strongly inhibits the incorporation of amino acids into protein of tissue slices. The results presented in this paper show that this somewhat peculiar succinate effect, which is present in regenerating liver and in a well-differentiated hepatoma and absent in two anaplastic hepatomas, is well-marked in all stages of hepatic carcinogenesis by N,N'-dimethyl-4-aminoazobenzene. A working hypothesis is that the inhibition by succinate of the amino acid incorporation into protein is mediated by a shift of the redox level in the cell toward a more reduced condition.