Grass hay produced in the Coeur d'Alene River Basin of northern Idaho was fed to a group of 4 ponies. The hay contained Pb in concentration of 423 +/- 82 mg/kg and Cd in concentration of 10.8 +/- 1.4 mg/kg, resulting in daily exposures of the ponies to approximately 7.4 mg of Pb/kg and 0.19 mg of Cd/kg/day. The results in this group of ponies were compared with those from a group fed noncontaminated grass hay and given a daily dose of 10 mg of Pb/kg of body weight, in the form of lead acetate. Clinical toxicologic signs, hematologic changes, and blood and tissue Pb concentrations were similar in the 2 groups. However, the severity of the disease process appeared to be greater in the ponies fed the Pb- and Cd-contaminated hay. This was shown clearly by the shorter interval between onset of clinical changes and death in the ponies fed contaminated hay. The possibility of multiple heavy metal effects is discussed. Clinical toxicologic signs observed include incoordination, labial paresis, pharyngeal paresis, CNS depression, anorexia, and body weight loss. Anemia or marginal anemia was common and was often accompanied by the appearance of nucleated RBC and Howell-Jolly bodies in peripheral blood. Neither the hematologic response nor the blood Pb concentrations were reflective of the severity of poisoning, although blood Pb concentrations were greater than 0.35 micrograms/ml once clinical signs of toxicity were observed. Liver, kidney, spleen, brain, and bone Pb concentrations and liver, kidney, and brain Cd concentrations were increased in both the ponies fed contaminated hay and the ponies given lead acetate.