We have compared the physical properties of two commercial emulsion formulations of the intravenous anaesthetic propofol, (Diprivan, AstraZeneca, and Propofol Intravenous Emulsion, Gensia Sicor Pharmaceuticals) which appear to differ primarily in the additive content and formulation pH. Diprivan contains disodium edetate and has a pH of 7-8.5, while the Gensia product contains sodium metabisulphite and is formulated to a pH of 4.5-6.4. The average zeta potential of Diprivan at pH 8 was -50 mV while that of the Gensia product at pH 4-5 was -40 mV. This information suggests that the physical stability of Propofol Intravenous Emulsion should be lower than that of Diprivan. Three random batches of both products were subjected to a range of stability tests, including shaking, thermal cycling, and freeze-thaw cycling, and the emulsion droplet size distribution was then assessed by dynamic light scattering, light diffraction, and electrical and optical zone sensing. Both emulsions initially showed narrow submicrometre particle size distributions. An increased level of droplets larger than 5 microm could be detected in Propofol Intravenous Emulsion after as little as 4 h shaking (300 strokes/min at room temperature) and visible free oil could be detected after 8-12 h shaking. In contrast, Diprivan showed no increase in the large droplet count after shaking for times up to 16 h. A similar difference in the emulsions was found after one freeze-thaw cycle, with Propofol Intravenous Emulsion exhibiting extensive coalescence, while that of Diprivan was at the limits of detection. We conclude that these two products have different physical stability characteristics, and that this may in part be due to the reduced zeta potential in Propofol Intravenous Emulsion compared to that of Diprivan.