The kinetics of procainamide N-acetylation were studied in the rat in vivo and in vitro. For the in vivo studies, first order kinetics for procainamide and N-acetylprocainamide were found among their respective iv doses of 20, 50, and 70 mg/kg, and 10, 20, and 30 mg/kg. The fraction of total body clearance of procainamide that forms N-acetylprocainamide was found to be 0.22, and very insignificant sequential elimination of N-acetylprocainamide occurred during its formation. By contrast, results from once-through liver perfusion indicated that the steady state hepatic extraction ratio of procainamide was highly dependent on the steady state input concentration delivered under constant hepatic blood flow (10 ml/min). The rate of N-acetylation, however, was a constant percentage of the rate of presentation of procainamide at less than or equal to 80 micrograms/min among the preparations and became apparently saturated at higher rates of input of procainamide. Interestingly, the rate of N-acetylation accounted for an increasing proportion of the rate of loss of procainamide at greater than 80 micrograms/min, and suggested that alternate metabolic routes of procainamide are more easily saturable than N-acetylation. The comparative in vivo and in vitro data suggested that a region of nonlinearity existed during the early periods immediately following iv injection of procainamide into the rat in vivo. Because of rapid distribution, the region of nonlinearity was transient, and was not reflected by area under the curve measurements, which is a time-averaged parameter. Total body clearance, which bears a reciprocal relationship with the area under the curve, hence remained constant and was dose-invariant. The trend of nonlinearity may be more evident on chronic dosing of the drug when accumulation sets in.