After institutional approval and with written informed consent, eight surgical patients were infused intravenously with remifentanil at 250 ng.kg lean body mass (LBM) ^-1 .min ^-1 for 30 min. Cardiovascular and respiratory parameters were recorded and arterial blood samples were taken at regular intervals. In each patient, the same protocol was repeated 40 min later during propofol infused to a target concentration of 3.0 μg.ml ^-1. Blood concentrations of remifentanil and propofol were assayed using capillary gas chromatography and high performance liquid chromatography techniques respectively. The number of subjects enrolled was determined by testing the successive areas under the remifentanil time-concentration curve (AUC) for significant difference or non-difference using sequential analysis. The median measured propofol concentration was 3.5 (range: 2.6–4.5) μg.ml ^-1 which did not change significantly during the second remifentanil infusion. The median AUC during propofol infusion was greater than control in all subjects, although there was considerable variation of 94.4 (64.3–129.6) versus 64.6 (34.8–126.9) ng.ml ^-1 .min; P=0.008, n=8. After 30 min, there was no significant difference in remifentanil concentration during propofol infusion when compared with remifentanil alone of 4.6 (3.2–5.7) versus 3.8 (1.6–4.9) ng.ml ^-1; P=0.73, n=8. Co-administration of propofol and remifentanil may result in greater remifentanil concentrations than when remifentanil is infused alone.