The regulation of erythrocyte synthesis of nicotinate and adenine nucleotides has been investigated. Some effectors of the two committed enzymes, nicotinate- and adenine phosphoribosyltransferases, have been identified on crude lysates and on partially purified preparations of the former. Enzyme characteristics have been correlated with the nucleotide synthesis achieved in intact cells incubated in suitable mediums containing (14-C)-nicotinate or adenine. Inorganic phosphate, Mg ions and adenine nucleotides proved to be important effectors of pyridine synthesis, whose products, in turn, do not influence adenine nucleotide production. The production of pyridine nucleotides is little lower than that of adenine nucleotides in intact cells, even if adenine phosphoribosyl-transferase activity appears to be much more limited, inside the cell, than nicotinate phosphoribosyltransferase.