On the basis of a study of the action of phosphoribosylpyrophosphate (PRPP) synthetase (EC 2.7.6.1.) in 22 hyperuricemic and 9 normal subjects, the authors report two cases of gout with anomalies of the kinetics of this enzyme. The anomaly consists of an increase in affinity for inorganic phosphate (iP) in the absence of inhibitors, and a decrease in inhibition by low concentrations of ADP in the presence of iP. These patients show no other anomaly of intraerythrocyte PRPP (dosage and production "in vitro"), hypoxanthine-guanine-phosphoribosyl-transferase and APRTase (overall activity and apparent Km). These two cases of gout are characterized clinically by their early occurrence, appearing in one case in a pre-menopausal woman, their family character and their normal sensitivity to allopurinol.