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The increase in remifentanil concentration during haemorrhagic shock and the difference between this effect and that for propofol are not fully understood. We investigated the influence of haemorrhage on the pseudo-steady-state remifentanil concentration in a porcine model and compared the changes with those for propofol.After infusion of remifentanil (0.5 µg kg−1 min−1) and propofol (6 mg kg−1 h−1 after 2 mg kg−1 bolus infusion) for 60 min, nine swine [mean (standard deviation) body weight=26.3 (1.3) kg] were studied using a stepwise haemorrhage model (10% of estimated blood volume removed every 30 min until 1.5 h, and stepwise removal of 5% every 30 min thereafter until circulatory collapse). Haemodynamic and metabolic variables and plasma remifentanil and propofol concentrations were measured at every step.A mean volume of 913 (82) ml of blood was drained before reaching circulatory collapse. The increases in plasma concentrations from the prehaemorrhagic value fitted the following equations: % increase in remifentanil=2.1×cumulative blood loss (% of initial blood volume) and % increase in propofol=0.7×cumulative blood loss during compensated shock; and % increase in remifentanil=27.4×cumulative blood loss−897 and % increase in propofol=9.5×cumulative blood loss−306 during uncompensated shock. Remifentanil concentrations were highly correlated with the reciprocal of cardiac output.During haemorrhage, the plasma remifentanil concentration showed a three-fold greater increase than that of propofol in administration by continuous infusion.