Continuous infusion of interpleural bupivacaine maintains effective analgesia after cholecystectomy. Twenty-five patients who had undergone elective cholecystectomy were prospectively randomized to receive via an interpleural catheter either a continuous infusion of 0.25% bupivacaine at 0.125 mL.kg-1.h-1 (n = 13) or repeated bolus injections (n = 12) of 0.5% bupivacaine with epinephrine 1:200,000 at 0.4 mL/kg every sixth hour. Adequacy of pain relief was measured by the amount of patient-controlled analgesia morphine required postoperatively and by patient scores on a visual analog scale obtained every sixth hour. Two venous blood samples for measurements of serum bupivacaine levels were obtained from patients in the continuous group at hours 6 and 24; four blood samples were obtained from patients in the bolus group, both immediately before and 30 min after injections at hours 6 and 24. Among the patients receiving the bolus injections, morphine was required 62 +/- 15 (SEM) times over the 24-h study period with total morphine dosage averaging 30 +/- 15 mg. Corresponding values for patients in the continuous groups were 35 +/- 10 times and 23 +/- 5 mg of morphine. The difference was not, however, statistically significant, but when activity during the 2-h time periods immediately before reinjection were examined, patients in the bolus group required and received significantly more morphine than did those in the continuous group (P less than 0.05). Patients in the continuous group had visual analog scale scores that averaged 2.9 +/- 0.6 over the 24-h study period. Patients within the bolus group had visual analog scale scores before and again 30 min after injection that averaged 5.8 +/- 0.8 and 1.8 +/- 0.5, respectively (P less than 0.05). 