The physicochemical compatibility of propofol and thiopental sodium when mixed together in various ratios and stored was studied. Mixtures of propofol and thiopental sodium in five volume ratios from 1:3 to 3:1 were refrigerated (4 degrees C) for up to seven days and then centrifuged at 2000g for two hours. Droplet sizes were determined at intervals by optical microscopy and laser diffraction, and chemical stability of the 1:1 mixture was evaluated by high-performance liquid chromatography (HPLC). Optical microscopy and laser diffraction indicated negligible changes in droplet size within 48 hours of mixing. A small increase in the width of the frequency distribution of droplet sizes occurred 24-48 hours after mixing for the two mixtures with the lowest propofol concentration. Some coalescence of droplets occurred on centrifugation. These results indicated negligible formation of droplets that might cause embolism after i.v. injection of fresh mixtures (not more than six hours old). A yellow color appearing after 24-48 hours indicated anticipated chemical changes. HPLC of samples stored at 25 degrees C indicated clinically unimportant drug loss after six hours. The mixtures were considered physically stable for not more than 48 hours. Droplet size in mixtures of propofol and thiopental sodium did not increase until at least 24 hours. Drug loss from mixtures containing propofol 5 mg/mL and thiopental sodium 12.5 mg/mL was insignificant for up to eight hours.