A total of 31 pediatric cardiac patients (mean age 19 1/2 months) who required surgery for repair of various congenital heart defects were prospectively studied in the postoperative period to determine potassium (K) dose-response characteristics. All patients received supplementary K when the serum K was less than 4.0 meq/liter. A total of 100 administrations of intravenous potassium chloride (KCl) were evaluated. KCl, 0.5 meq/kg, was infused over 2 h by syringe pump. This infusion was repeated as necessary to achieve a serum K greater than or equal to 4.0 meq/liter. The KCl was administered in addition to the K in the patients' maintenance solution. Blood samples for serum K determination were collected 15-30 min before and after the KCl infusion. The mean K dose administered was 0.72 +/- 0.23 meq/kg. This produced a mean rise in serum K of 0.61 +/- 0.48 meq/liter. Of 100 administrations, 11 resulted in no change or a decrease in serum K. Four of 100 administrations resulted in serum K greater than 5.0 meq/liter. Intravenous KCl supplementation in a dose of 0.5 meq/kg administered over 2 h is safe and effective for pediatric postoperative cardiac patients. Serum K should be measured to monitor therapy, due to variable response.