Holter electrocardiographic recordings were performed for 24 hours in 20 patients with chronic renal failure on chronic hemodialysis to evaluate the effects of changes in serum calcium and potassium during hemodialysis period on the Qr interval. Hemodialysis caused an increase in serum calcium from a predialysis value of 9.1 +/- 1.3 mg/dl to 11.5 +/- 1.2 mg/dl and a decrease in serum potassium from 5.6 +/- 1.4 mEg/L to 4.9 +/- 1.2 mEg/L. The Q-oTC interval shortened from a predialysis value of 0.240 +/- 0.0023 sec to 0.216 +/- 0.024 sec during the 5th hour of hemodialysis. The Q-eTc interval increased from a predialyse value of 0.391 +/- 0.030 sec to 0.412 +/- 0.024 sec during the 5th hour of hemodialysis. This shortening of Q-oTc interval was correlated with an increase in serum calcium and Q-eTc interval prolongation was correlated with a decrease in serum potassium. It was concluded that hemodialysis caused a lengthening of the Q-eTc interval and a shortening of the Q-oTc interval.