This study reports the development of a computer-based infusion system and methodology to produce and maintain selected plasma concentrations. The method identifies pharmacokinetic infusion parameters for subjects from bolus injection response data, employs these values in control equations implemented by a portable microcomputer and computer-controlled infusion pump, and achieves and maintains selected stepwise drug levels by intravenous drug infusion. Infusion studies with four dogs and five humans resulted in correlation coefficients of 0.98 for the dogs and 0.94 for the humans, with rms errors in maintaining the drug concentration at the desired level of 13.4 and 19.3 percent, respectively. An analysis of error demonstrated that: 1) the control error was less than the value of the pharmacokinetic parameter estimation error in determining a single parameter value, 2) errors in several parameters can have cancelling or additive effects depending on their sign, and 3) an error in the sum of two of the model parameters (A and B) directly translates to equivalent time-independent error in the controlled level.