A combination of meperidine (M) 25 mg/ml, promethazine (P) 6.5 mg/ml, and chlorpromazine (C) 6.5 mg/ml is widely used to produce sedation in pediatric patients. A dose of MPC 0.1 ml/kg is recommended for cardiac catheterization, but no specific guidelines for dosing or frequency of monitoring have been established for patients undergoing other types of procedures. The adverse effects of MPC were studied prospectively in 95 patients undergoing various procedures. MPC was given parenterally at a dose of 0.07-0.11 ml/kg. Four patients developed respiratory depression. In these patients, the lowest respiratory rate ranged from 12 to 20 per minute. The lowest pulse rate ranged from 92 to 102 per minute. Three patients had received recommended or lower than recommended doses of MPC. One who received MPC 0.07 ml/kg developed respiratory arrest within 30 minutes; another required naloxone, and all recovered within 10 hours. These cases suggest the need for frequent monitoring and specific dosing guidelines for MPC use in pediatric patients.