The intravenous disposition of gentamicin was compared in the conscious chronically catheterized rat with that in rats anesthetized using five injectable laboratory anesthetics. Gentamicin plasma clearance in the conscious rat was significantly higher than in animals anesthetized with urethane, fentanyl/fluanisone/midazolam, pentobarbitone, or ketamine/midazolam but similar to that in rats anesthetized with alphaxolone/alphadolone. Urethane anaesthesia resulted in a significantly lower gentamicin clearance than in all other groups. Gentamicin clearance in rats anesthetized with alphaxolone/alphadolone was significantly higher than in rats anesthetized with either fentanyl/fluanisone/midazolam or urethane. No significant differences in the volume of distribution of gentamicin were observed between any of the groups studied, either anesthetized or conscious. Carboxyinulin blood clearance in the conscious group was significantly higher than that with urethane, fentanyl/fluanisone/midazolam, pentobarbitone, or ketamine/midazolam but not significantly different from alphaxolone/alphadolone-anesthetized animals. The differences in carboxyinulin clearance were noted to be proportional to the differences in gentamicin clearance (r2 = 0.98). These results demonstrate that the choice of anesthetic used in laboratory pharmacokinetic studies is important. Gentamicin clearance was higher in conscious than anesthetized rats, and it may be prudent to use chronically catheterized animals in pharmacokinetic studies.