The pharmacokinetics following a bolus injection of fentanyl 60 micrograms kg-1 as the sole anaesthetic in five patients undergoing cardiac surgery are described. Before cardiopulmonary bypass the kinetics can be described by a biexponential decay curve with an initial half-life of 1.7 +/- 0.85 min and a half-life during the tissue uptake phase of 69 +/- 8.2 min. Two patients had secondary peaks in plasma fentanyl concentration during the first 5 min after injection. After 5 min cardiopulmonary bypass with haemodilution, the fentanyl concentration decreased by an average of 53%. The half-life in the post-bypass elimination phase was 423 +/- 36.9 min. No second peaks during the elimination phase were found.