The comparative pharmacokinetics of Aspegic, the lysine salt of acetylsalicylic acid, administered in multi doses either through i.v. or i.m. route was studied. 1 g of drug was injected each time with a frequency of 3 times a day. The pharmacokinetic parameters were determined using the experimental data in the literature. From these results, three categories of patients were considered, depending on their response to the drug. A numerical model was established in order to evaluate the following results: the drug level in the blood compartment obtained with the i.v. or i.m. administration, as well as the area under the curve for the first day and the third day when the so-called stationary state was obtained. Approximately similar values for AUC were obtained for each route of administration, for a given category of patients. The effect of the inter-variability of the patients characterised by their response to the drug was found to be of prime importance.