The amplitude of the QRS wave of the electro-cardiogram was correlated with conventional serum electrolyte concentrations in 94 nonhypertensive and 53 hypertensive patients. There was a highly significant correlation of QRS amplitude with serum albumin level in both groups. There was also significant correlation with serum calcium level in the nonhypertensive group, but this significance disappeared after correction for albumin. Changes in the serum albumin level during the hospital admission of the nonhypertensive patients were positively correlated with changes in QRS amplitude. Albumin infusion into 13 healthy persons resulted in significantly increased QRS amplitude, which was related to changes in serum albumin concentration. These results suggest that changes in serum protein concentration cause changes in QRS amplitude, possibly as a result of increased conductivity.