Gentamicin blood levels and pharmacokinetic parameters were studied in 14 horses after intramuscular administration (musculus serratus ventralis) of two doses, 3 and 6 mg/kg of body weight. Microbiological cylinder plate assay was used to determine gentamicin serum concentrations, using Staphylococcus epidermidis (ATCC 12228) as a test organism. Samples were collected 5, 10, 15, 20, 30, 45, 60, 120, 240, 360 and 720 minutes after antibiotic administration. The results were best fitted to a one compartment model with a constant of absorption (ka) of 5.14 +/- 1.56 h(-1) and 5.48 +/- 0.75 h(-1) elimination half-life (t) of 4.58 +/- 0.31 h and 4.60 +/- 0.92 h, volume of distribution (Vd) 0.26 +/- 0.13 1*kg(-1) and 0.42 +/- 0.16 1*kg(-1) and total body clearance (Cl-total) of 0.64 +/- 0.35 ml*min(-1) kg and 1.06 +/- 0.34 ml*min(-1)*kg(-1) for doses of 3 and 6 mg/kg, respectively. Absolute bioavailability was 101.48 +/- 10.49% and 93.98 +/- 19.48%, C-max was 10.96 +/- 6.20 mu g/ml and 12.10 +/- 8.78 mu g/ml and t(max) was 42.36 +/- 13.55 min and 40.44 +/- 12.63 min for the low and high dose, respectively. Results showed that serum concentrations over 1 mu g/ml persisted up to twelve hours in animals treated intramuscularly. It was concluded that intramuscular administration of doses between 2.5 and 3.0 mg/kg every 12 hours are adequate to maintain therapeutic serum concentrations of gentamicin in the horse.