The influence of elevated dietary phosphate on the toxicity of orally ingested lead was investigated in male weanling Wistar rats. Two groups of 20 rats were fed diets containing either adequate (0.5%) or high (1.2%) levels of phosphorus (as phosphates). Half of the rats on each level of phosphorus were given 20 micrograms lead (as lead acetate)/g dry diet. After 8 wk, biochemical tests for lead toxicity were carried out and tissue-lead levels were measured. All of the rats given lead-supplemented diets had higher concentrations of lead in bone, brain, kidney and liver than those given diets without added lead but the increase was significantly greater in all tissues in the group given the 1.2% phosphorus diet. Lead supplementation increased the levels of free erythrocyte protoporphyrin and decreased the activity of delta-aminolaevulinic acid dehydratase. The lead-induced increase in the level of free erythrocyte protoporphyrin was no greater in the high-phosphate group than in the low-phosphate group but the decrease in delta-aminolaevulinic acid dehydratase activity was significantly larger in the high-phosphate group. This work demonstrates that excess dietary phosphate significantly increases lead toxicity and indicates the need for continued research on the interaction between dietary factors and lead toxicity.