The pharmacokinetic behaviour of gentamicin sulphate (3.4 mg/kg bwt) was studied following its intramuscular injection to a group of horses and to another group of horses premedicated with sodium methyl arsinate (2 mg/kg bwt) or imidocarb dipropionate (4.8 mg/kg bwt). Considerable differences were observed in the pharmacokinetics of gentamicin in pre-medicated horses and in horses which had received the antibiotic alone. Peak serum concentration of gentamicin (9.85 +/- 0.05 and 11.15 +/- 0.15 micrograms/ml) were attained within 1.45 +/- 0.05 and 0.92 +/- 0.04 h in arsinate and imidocarb-medicated horses, respectively, but reached a level of 9.18 +/- 0.07 micrograms/ml at 1.87 +/- 0.12 h in non-medicated animals. Gentamicin elimination half-life (+/- 0.5(el)) was faster in arsinate (7.82 +/- 0.31 h) and imidocarb (6.12 +/- 0.14 h) pre-medicated horses than in non medicated horses (9.88 +/- 0.19 h). In addition, the interval between doses (lbd) necessary to maintain a therapeutic level were shorter in the pre-medicated animals. In summary, the hypothesis that the pharmacokinetics of gentamicin are altered by concomitant therapy with antiprotozoal drugs was confirmed by this study.