BACKGROUND: Both propofol and ketamine are useful hypnotics for induction of anesthesia, and the combination of propofol and ketamine has been used for total intravenous anesthesia. The aim of this study was to evaluate the dose response of propofol, ketamine and combination of these drug, and determine possible interaction between two drugs in patients. METHODS: The effect of ketamine on the dose response curve for propofol was studied in unpremedicated 165 ASA physical status I or II patients who were scheduled for elective operation. As an endpoint of hypnosis, ability to open eyes on verbal command was checked. Dose response curves for propofol and ketamine were determined with a probit procedure and their type of pharmacologic interaction was determined by fractional and isobolographic analysis. RESULTS: At the hypnotic endpoint, the ED50s were 1.13 mg/kg propofol, 0.66 mg/kg ketamine, and the ED95s were 1.67 mg/kg propofol, 1.09 mg/kg ketamine. The type of interaction between two drugs for hypnosis was found to be additive and ketamine was 1.7 times potent than propofol as an equieffective dose of hypnosis. CONCLUSIONS: The type of interaction between propofol and ketamine for hypnosis was additive.