An aqueous solution of ampicillin sodium (100 mg ml-1) was administered intravenously and intraosseously to six kitten at 50 mg kg-1 in a crossover study. Jugular vein blood samples were taken at intervals up to eight hours after treatment and the serum ampicillin concentration-time data, derived from a microbiological assay, were analysed pharmacokinetically. The disposition kinetics of ampicillin administered by the two routes were very similar. The mean elimination half-life (t i 2β), the area-derived volume of distribution (Varea) and the total body clearance (ClB) values after the intravenous and intraosseous treatment were 86·3 and 79·0 minutes, 0·9 and 0·8 litre kg-1 and 7·3 and 7·6 ml min -1 kg-1, respectively. No side-effects related to the intraosseous administration of the drug were observed.