A mass-balance study was undertaken to evaluate the accuracy of dose determinations of waterborne 2,5,2',5'-[14C]tetrachlorobiphenyl (TCB) made on transected brook trout (Salvelinus fontinalis) and rainbow trout (Salmo gairdneri) and to determine any pharmacokinetic differences between the two species. The total calculated [14C]TCB absorbed by brook (17.8 micrograms) and rainbow (24.5 micrograms) trout was compared to the actual body burden measurements of [14C]TCB for brook (17.4 micrograms) and rainbow (25.6 micrograms) trout; the latter measurements also included excretory losses through the urine, feces, and across the gill surface. Approximately 1% of the total dose was excreted of which 75% was in the urine and 25% in the feces. The agreement between the whole body burden measurements of [14C]TCB and the total calculated micrograms of [14C]TCB absorbed was within 10% in both species. Mass-balance measurements were converted to dose by dividing by fish weight. Mean calculated and measured doses were 31.4 and 30.4 micrograms/kg/48 hr for brook trout and 32.3 and 33.6 micrograms/kg/48 hr for rainbow trout. No species differences were seen in either calculated or measured doses or in total radioactivity excreted. Respiratory function between the two species was similar except for a significantly higher mean ventilation rate for brook trout (84 +/- 14/min) than for rainbow trout (65 +/- 4/min). There were also no fluctuations noted in either respiratory function of [14C]TCB uptake efficiency across the gills over the 48-hr exposure period. These studies demonstrated the ability to accurately calculate a water dose in micrograms per kilogram per hour for individual fish that could be directly compared to other fish species or to mammals.