Gentamicin blood levels and pharmacokinetic parameters were studied in 14 horses after intravenously administering doses of 3 and 6 mg/kg of body weight. The microbiological cylinder plate assay, using Staphylococcus epidermidis (ATCC 12228) as the test organism was used to determine gentamicin serum concentrations. Samples were collected at 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, 360 and 720 minutes after antibiotic administration. The results best fitted an open two compartment model with central volume of distribution (Vd((c))) of 0.13 +/- 0.16 and 0.19 +/- 0.07 L*kg(-1), elimination half-live (t1/2 beta) of 3.43 +/- 0.84 and 3.78 +/- 0.48 h and total body clearance (Cl-(Total)) of 0.64 +/- 0.54 and 0.99 +/- 0.75 L*min(-1)*kg(-1) for doses of 3 and 6 mg/kg, respectively. Results showed that serum concentration of 1 mu g/mL persisted up to twelve hours in animals treated intravenously. It was concluded that gentamicin has an adequate Vd((c)) in the treatment of systemic infections caused by sensitive bacteria and that adequate serum and tissue concentrations of gentamicin are reached rapidly after intravascular administration.