Twenty-nine patients (age range 14-81 yr) undergoing orthopaedic surgery received alfentanil 100 micrograms kg-1 given as two i.v. boluses followed by a fixed rate infusion of 1 micrograms kg-1 min-1 for 44-445 min. Additional 1-mg bolus doses of alfentanil were administered as required. Plasma samples were assayed for alfentanil using radioimmunoassay. Pharmacokinetic parameters were estimated by a model-independent approach and by curve-fitting. Regression analysis showed no statistical relationship between T 1/2 beta, Cl or Vd and the duration of the infusion, total dose or body weight. We found no significant correlation between age and T 1/2 beta of alfentanil for patients younger than 40 yr. For patients older than 40 yr, T 1/2 beta increased linearly with age. There was no significant decrease in Cl with age, although the lower values for Cl (100-200 ml min-1) were generally found in subjects older than 60 yr. The present study demonstrated that a 100-micrograms kg-1 loading dose and a 1-micrograms kg-1 min-1 infusion may be appropriate for analgesia in general surgical procedures.