Gentamicin pharmacokinetics has not been studied in horses. Pharmacokinetics of gentamicin C1, C1a and C2 components following i.v. administration of total gentamicin at 6.6 mg/kg bwt to 6 healthy mature horses was determined. Significant differences in clearance, half-life (t 1/2), and mean residence time (MRT) between the gentamicin Cia and the 2 other components were found. The total body clearance (CL) of gentamicin C1a was 1.62 +/- 0.50 ml/min x kg and similar to the glomerular filtration rate (GFR) reported for horses. The CL of gentamicin C1 and C2 were 1.03 +/- 0.08 ml/min x kg and 1.10 +/- 0.15 ml/min x kg, respectively, and significantly slower than that of gentamicin C1a. The values of apparent volume of distribution at steady state were 0.22 +/- 0.05, 0.26 +/- 0.12 and 0.23 +/- 0.05 l/kg for gentamicin C1, C1a and C2, respectively. The MRT values were mean +/- s.d. 3.6 +/- 0.5, 2.7 +/- 0.3 and 3.5 +/- 0.4 h and the t 1/2 values were 3.1 (2.5-4.0), 2.4 (2.0-3.2) and 33 (2.4-4.3) h (harmonic mean and range) for gentamicin C1, C1a and C2, respectively. The MRT and t 1/2 values for gentamicin C1a were significantly shorter than those of gentamicin C1 and C2. It was concluded that the difference in pharmacokinetics between the gentamicin components has potential pharmacological and toxicological implications.