The medical necessity for continuous injection of anesthetic and analgesic drugs still presents unresolved problems. The principal feature of our infusion management system CARIN (Computer Assisted Regulation of an INtravenous INjection) described in this paper is that it allows desired plasma concentrations to be established and maintained with great precision (within 1%). The CARIN system allows several types of user intervention, to raise or to lower plasma concentration, to suspend injection temporarily followed by an automatic return to the preceding concentration, to inject a bolus or to carry out manual injection with subsequent adjustment of the regimen. Plasma concentration may be varied at will, either very swiftly (spontaneous when descending) or gradually over a specified time. Given the current lack of on-line methods for measuring drug concentrations in plasma, a mathematical model is used to simulate evolution of the situation. The CARIN system may be used for all substances whose pharmacokinetic behaviour has been modelled, whether the model has two or three compartments. It allows individual adjustment of drug injection according to the characteristics of the patient. In addition, it replies interactively to the questions which the anesthetist may wish to ask about the progress of the drug infusion.