The pharmacokinetics off cefoxitin were studied after a single i.v. and i. m. dose of 15 mg/kg of cefoxitin to 15 younger people (15-55 years) and to 16 geriatric patients (66-94 years). The kinetics of the antibiotic follow an open two-compartment model for both administration routes. After i.m. administration of the antibiotic, there are modifications in the distribution with respect to i.v. administration, and there is a decrease in beta K21 and as a consequence, an increase in Vp. In geriatric patients, pharmacokinetic modifications take place with respect to those recorded in the younger people in both administration routes, there being a decrease in the following pharmacokinetic parameters: alpha, beta, K12, K21, K13 and Clp, together with an increase in the apparent distribution volume Vdss. A linear relationship has been established between the constant beta and the age of the patient. In spite of the pharmacokinetic modifications observed as a function of age, given the wide safety margin of the antibiotic, it is not necessary to modify the dosage regimen of cefoxitin in geriatric patients.