The aim of the present study was to develop the aqueous parenteral formulation containing propofol using o/w microemulsion systems. Propofol itself was chosen as the oil phase and its content was fixed to 1%, w/w. Pseudoternary phase diagrams were constructed to obtain the concentration range of surfactant and cosurfacatnt and the optimum ratio between them for microemulsion formation. Consequently, the suitability of the chosen microemulsion system as a parenteral formulation was evaluated from the stability and hemolysis tests on that. Among the surfactants and cosurfactants screened, the mixture of Solutol HS 15-ethyl alcohol (5/1) showed the largest o/w mocroemulsion region in the phase diagram. When 1% (w/w) of propofol was solubilized with 8% (w/w) of Solutol HS 15-ethyl alcohol (5/1), the average droplet size (150 nm) and the content of propofol in the systems were not significantly changed at 40 degrees C for 8 weeks. The hemolysis test showed that this formulation was nontoxic to red blood cells. In conclusion, propofol was successfully solubilized with the o/w microemulsion systems.