The pharmacokinetic behaviour of cephalosporin antibiotic, second generation after intravenous administration to patients during the early post-operative period was characterized as well as after cholecystectomy. The data suggest a two-compartment distribution with elimination from the central compartment, realized on account of the renal way of clearance. In one of the patients, with one-compartment distribution of the drug, the presence of the theoretical determining factors in "the collapse" of one two-compartment open model to one-compartment is shown, associated with clinical characteristics of that patient.