The hypnotic effects of thiopentone, propofol and their combination were studied in 120 unpremedicated ASA group I patients. The end-point for induction of anaesthesia was taken as inability to open the eyes to command 60 s after the end of injection. The dose-response curves were determined by probit analysis. Isobolographic and algebraic (fractional) analyses were used to assess quantitatively the combined hypnotic effect of thiopentone and propofol and to define the type of interaction between these two drugs. The interaction between thiopentone and propofol was found to be synergistic.