The effect of food intake versus brief fasting on gastrointestinal absorption of lead was measured in five healthy men who were living in a metabolic unit and eating constant lead diets. Lead absorpiton was assessed by the difference between dietary intake and output of 1) lead tracers composed of nonradioactive isotopes which were ingested as a single dose either with food or during a 16-hr fast, 2) lead tracers ingested with meals for relatively long periods (2 to 124 days), and 3) total led in ingested foods. Absorption estimated by 1) was confirmed by increments in tracer concentrations in blood. Lead tracers were given as nitrate, cysteine complex, or sulfide. Absorption was 10.3 +/- 2.2% (SD) for food lead; 8.2 +/- 2.8% for tracers ingested with food; and 35 +/- 13% (P < 0.01) percent for tracers ingested without food. The increased absorption of lead when ingested without food should be considered when the hazards of exposure to lead are determined.