A method and apparatus for controlling a model based open-loop process and more specifically, the concentration of a drug delivered intravenously to a patient as a function of the rate of infusion. A three-compartment pharmacokinetic model is preferably used to determine the plasma drug concentrations corresponding to two arbitrarily selected rates of infusion. Based upon the linear relationship between corresponding data pairs, each pair comprising a rate of infusion and a corresponding plasma drug concentration, an interpolated rate is determined by a microprocessor (100) as a function of a specified plasma drug concentration, in accordance with program steps stored in a read only memory (ROM) 104. The actual infusion rate of the drug during successive time intervals is repetitively used to compute the plasma drug concentration at the end of each time interval. In each successive iteration, state variables from the previous computation of the plasma drug concentration are used to determine the next interpolated infusion rate. Any interruptions in the output from an infusion device (112) and variations from the interpolated value of the infusion rate are compensated. Using this open-loop control method and apparatus, the specified plasma drug concentration is rapidly achieved.