A regime for administration of ampicillin sodium by continuous intravenous infusions to horses was designed. The aim was to achieve plasma ampicillin concentrations between 5 and 10 pgiml over a 4‐h period. A 2 mgikg bodyweight loading dose of ampicillin sodium was administered intravenously at the beginning of the infusion in order to achieve steady‐state plasma concentra‐tions rapidly. The infusion system subsequently administered ampicillin at a rate of approximately 19.2 pglminikg bodyweight. The plasma concentrations obtained over the infusion period correlated very well with predicted calculations based on pharmacokinetic parameters. A mean ± SEM steady‐state plasma concentration (Cpss) of 5.94 ± 0.33 was obtained and ampicillin was shown to have an apparent steady‐state volume of distribution (Vdss) of 175.43 ± 13.63 ml/kg. When the pump was disconnected the concentrations declined over the following 4 h in an exponential way with an elimination half‐life (t1/2β) of 0.62 h. In addition, three different infusion dose rates (13.78, 19.34 and 2 l 4 8 pg/min/kg) were administered to a single animal showing that a good correlation(correlation coefficient>0.99) existed between the dose administered the steady‐state plasma concentrations and the corresponding areas under the plasma concentration