The concentrations of trimethoprim (TMP) and sulfadiazine (SDZ) in serum, bile and gallbladder wall of 9 patients with acute cholecystitis were measured after b.i.d. treatment with 160 mg of TMP + 500 mg of SDZ. The samples were collected 2-4 h after the last dose. Mean TMP concentrations were 1.71 +/- (SE) 0.51 micrograms/ml, 4.53 +/- 5.26 micrograms/ml and 2.31 +/- micrograms/g in serum, bile and gallbladder, respectively. Mean SDZ concentrations were 22.2 +/- 14.9 micrograms/ml in serum, 9.37 +/- 8.99 micrograms/ml in bile and 16.1 +/- 10.1 micrograms/g in gallbladder. The average ratios of bile-serum and gallbladder-serum concentrations were 2.64 and 1.35 for TMP and 0.42 and 0.73 for SDZ. There was a significant correlation between serum and gallbladder concentrations of both TMP and SDZ. No correlation existed between serum and bile levels.