Groups of 15 to 18 female weanling Long-Evans rats fed a rye-based diet low in lead (0.25 ppm) were exposed to 0.1, 1.0, and 5.0 ppm lead in drinking water. No suggestion of clinical lead toxicity was recognized. Systolic pressures were measured at 3-month intervals after weaning. The groups of lead-exposed animals had consistently and significantly higher average pressures than control animals, the increase approximating 15 mm Hg. With the lowest lead exposure (0.1 ppm), the increase in average pressure was gradual, being half minimal at 3 months and requiring 1 year to become maximal. After 1 year, half of these rats had pressures from 0 to 10 mm Hg above the control average; 40, 20, and 10% had pressures that were 20, 30, and 40 mm Hg, respectively, above the control average. Thus, rats exposed to lead in amounts comparable to the environmental exposure of many Americans had an average elevation in systolic pressure comparable to that of human beings with essential hypertension.