The pharmacokinetics of the enzyme inducer, phenobarbital, was evaluated in genetically obese and lean Zucker and lean Sprague-Dawley rats. The volume of distribution of phenobarbital in obese Zucker rats was larger (0.416 liter) than for lean Zucker (0.299 liter) or Sprague-Dawley rats (0.312 liter). Standardizing the volume of distribution for total body weight in the obese and lean Zucker and lean Sprague-Dawley rats resulted in similar volume terms. Intra- and inter-strain differences in phenobarbital clearance were observed between the obese and lean Zucker (11.9 and 14.1 ml/hr, respectively) and lean Sprague-Dawley rats (23.0 ml/hr). Greater differences in phenobarbital clearance were observed when clearance was corrected for body weight. Whether comparing the absolute or standardized pharmacokinetic data, lean and obese Zucker rats will exhibit 2- to 3-fold higher phenobarbital plasma concentrations after administration of a standard 75- to 100-mg/kg enzyme-inducing regimen relative to Sprague-Dawley rats. Pharmacokinetic parameters from the single dose study were used to calculate appropriate phenobarbital doses (21-58 mg/kg/12 hr) to achieve similar steady-state phenobarbital plasma concentrations after chronic oral administration in all three groups of rats. Steady-state phenobarbital clearance values were not significantly different from clearance values after single dose administration in each group of rats. The dramatic intra- and inter-strain alterations in phenobarbital disposition demonstrated in this study explain the high mortality reported in Zucker rats after administration of traditional enzyme induction doses of phenobarbital. Differences in phenobarbital disposition should be considered in enzyme induction studies.