In guinea pigs, administration of digitoxin (0.3 mg/kg s.c. for 7-24 days) causes an increase in activity of the (Na+ + K+)-ATPase of the heart. The plasma K+ level and the K+ content of the heart muscle of these animals remains unchanged and there is no significant alteration in the digitoxin toxicity compared to controls. In guinea pigs with potassium deficiency produced by a potassium deficient diet for 12 days, there is a related increase of the (Na+ + K+)-ATPase. The plasma K+ level of these animals is diminished while the K+ content in the heart muscle remains unchanged the toxicity of digitoxin is enhanced. In both test groups the increase in the (Na+ + K+)-ATPase activity is limited to enzymes from heart muscle, those from brain or kidney remaining unaffected. This increase in activity seems to be the result of an adaptive enzyme induction.