Juvenile chickens were either chronically exposed to 300 microgram/ml of Hg in the drinking water (chronic dosage--CD) or acutely exposed to either 3 mg of Hg/kg body wt (acute low dosage--ALD) or 12 mg of Hg/kg body wt (acute high dosage--AHD) administered intramuscularly for five consecutive days. Only the CD and AHD treatments retarded normal growth. Relative heart weights were increased by the CD treatment, decreased by the AHD treatment, and remained unchanged during the ALD treatment. Relative aorta weights were increased by the CD treatment but decreased by the AHD treatment. ECG analyses revealed a consistent decrease in the amplitude of the R-S and T waves with the greatest effects present in the acute (ALD and AHD) treatments. Histological examination revealed that Hg caused myocardial histopathological changes characterized as a myocarditis with polymorphonuclear and lymphocytic infiltration and fatty degeneration. It is concluded tha Hg causes cardiovascular disturbances in chickens even when administered at levels which do not inhibit normal growth.