Lead was administered to male and female rats in drinking water for 3 and 6 weeks at the following doses: 0, 10, 100, 1000, 5000 ppm and for 6 months at 10 ppm only. Various parameters of blood - lead concentration (Pb-B), hematocrit (Htc), hemoglobin (Hb), free erythrocyte porphyrins (FEB), delta-aminolevulinate dehydratase (ALAD), reticulocytes - and tissue - ALAD, free tissue porphyrins (FTP), lead concentration (Pb-T) - were determined. Pb-B increases with dose but reaches rapidly a plateau despite continuous Pb administration. Concentration of Pb in kidney, liver and brain correlates with Pb-B. Pb does not accumulate in heart. Kidney is the main site of Pb deposition and kidney ALAD is the parameter most susceptible to lead, since reduction is observed in all treated groups after 3 weeks of exposure. However, kidney ALAD inhibition is transitory since after 6 weeks it is only observed in the 5000 ppm group. At 10 ppm lead prevents also the increase in blood ALAD activity normally associated with the reticulocytosis of repetitive bleeding. The next parameters affected by lead are: ALAD in blood which is inhibited after 6 weeks of treatment with 100 ppm lead, and FEP, delta-aminolevulinic acid plus other pyrrole-forming substances in urine (ALA-U), and FTP in kidney which are increased after 3 or 6 weeks of treatment with 1000 and 5000 ppm lead.