The pharmacokinetic behaviour of an ampicillin/sulbactam (2:1) combination was studied after intramuscular administration of a single dose (20 mg/kg body weight: 13.33 mg/kg of ampicillin and 6.67 mg/kg of sulbactam) to six sheep and six goats. The objective was to determine whether there were differences between sheep and goats in the disposition profiles of ampicillin and sulbactam. The disposition curves for both drugs were best described by a biexponential equation (one-compartment open model with first order absorption) in both sheep and goats. The maximum plasma concentrations of ampicillin and sulbactam were similar in both sheep (10.61 +/- 6.36 and 9.17 +/- 3.82 mg/L respectively) and goats (11.02 +/- 2.69 and 9.25 +/- 0.85 mg/L respectively), in spite of the fact that the ampicillin dose was twice that of sulbactam. The time of the peak plasma concentration for both drugs was also similar in both sheep and goats. The elimination half-life of ampicillin was 0.81 +/- 0.17 h in sheep and 0.71 +/- 0.12 h in goats, and that of sulbactam was 1.02 +/- 0.36 h in sheep and 1.13 +/- 0.18 h in goats. The rate of drug removal from the body was faster in sheep than in goats and consequently the area under the curve was greater for goats. It was concluded that the similarity in the disposition and elimination of both drugs in sheep and goats indicated that the combination preparation could be administered at the same dosing rate in both species.