We compared the measurement of propofol concentrations in plasma or whole blood by high-performance liquid chromatography (HPLC) to that of gas chromatography (GC). Blood samples were collected from patients who had received bolus injection or continuous infusion of propofol. The results showed that the two methods correlated well both in plasma and whole blood samples. However, significant biphasic differences of propofol concentrations between plasma and whole blood specimens were observed in the bolus injection group. Differences were larger in the infusion group. This discrepancy in concentrations resulted from the infusion or clearance of propofol, and the lag of redistribution across blood cell membranes. In conclusion, monitoring of propofol concentrations by the methods of GC and HPLC gives equivalent results. For propofol concentration monitoring, plasma samples are preferred, but immediate centrifugation is needed.