Recent investigations have disclosed a decrease in pentose phosphate shunt activity in hereditary pyrimidine 5′-nucleotidase deficiency. Clinical lead poisoning is associated with an acquired decrease in pyrimidine 5′-nucleotidase activity. The current investigations were undertaken (1) to determine if pentose shunt activity was decreased in erythrocytes exposed to lead, and (2) to compare the mechanism of inhibition to that seen in hereditary pyrimidine 5′-nucleotidase deficiency. Normal erythrocytes incubated with lead acetate in vitro demonstrated increased Heinz body formation, decreased reduced glutathione, a positive ascorbate cyanide test, and a reversible suppression of pentose shunt activity in the intact erythrocyte. Lead acetate added to normal red cell hemolysates markedly inhibited the activities of glucose-6-phosphate dehydrogenase (G6PD) and phosphofructokinase. The mean Kis of lead for glucose-6-phosphate and nicotinamide adenine dinucleotide phosphate (NADP) for G6PD were 1.5 microM and 2.1 microM, respectively, which is within the range of intraerythrocytic lead concentrations found in clinical lead poisoning. Magnesium enhanced the ability of lead to inhibit G6PD. Thus, the shortened erythrocyte survival in lead poisoning appears to be due, in part, to increased oxidant sensitivity secondary to inhibition of G6PD and the pentose shunt. The mechanism of shunt inhibition is, in part, similar to that seen in hereditary pyrimidine 5′-nucleotidase deficiency.