A quantitative microdialysis (MD) sampling method was used to study phenol (PH) glucuronidation in vivo in rainbow trout. The method employs internal calibrators to account for changes in MD probe performance (in vitro-to-in vivo and sample-to-sample) and yields data of high temporal resolution that are well suited for developing kinetic models. Initially, trout were dosed with phenyl glucuronide (PG) by intravascular infusion for 24 h and then depurated for 48 h. Measured concentrations of PG in blood were well described by a one-compartment clearance-volume model. Mass-balance calculations showed that 93% of infused PG was eliminated in urine during the depuration period. Peak concentrations of PG in urine averaged 3.4 times higher than those in blood, and the fitted PG clearance constant (15.7 ml/kg/h) was about 2.6 times higher than the reported glomerular filtration rate for trout. These findings confirm earlier work suggesting that PG is actively secreted by the trout kidney. In a second set of experiments, trout were exposed continuously to PH in water. In vivo rate constants for PH glucuronidation were estimated using a pair of linked (PH and PG) one-compartment clearance-volume models. Expressed on a whole-fish basis, the glucuronidation rate averaged 0.049/h, which was about 7% of the total rate of PH elimination. This study demonstrates the utility of quantitative MD sampling for kinetic studies of xenobiotic metabolism in fish.