Inhibition by lead of erythrocyte pyrimidine 5'-nucleotidase (P5N) is thought to contribute to morphological abnormalities observed in red blood cells (RBC) of lead-exposed subjects. However, neither the mechanism of lead inhibition of P5N nor the relationship of this inhibition to blood lead levels attained in exposed subjects is known. In the present investigation, acute in vivo and in vitro lead acetate effects on erythrocyte P5N from 21-day-old rat pups were determined and were related to blood lead concentrations ascertained by atomic absorption spectrophotometry. Acute lead administration to rat pups resulted in a 16% to 21% reduction in erythrocyte P5N, with mean blood lead levels ranging from 77 to 108 micrograms/dl 24 hours later. Inhibition of erythrocyte P5N was linearly related to blood lead level (r = -0.67, P less than 0.05) following acute lead administration. Lead acetate addition to RBC preparations from 21-day-old rats resulted in concentration-dependent P5N inhibition which was comparable to that produced following acute in vivo exposure. The results indicate that acute P5N inhibition in lead-treated neonatal rats is due to noncompetitive P5N inhibition by lead. The inhibition of P5N produced by acute lead treatment is linearly related to blood lead concentrations.