Plasma concentration of gentamicin was measured 1, 4, and 6 hours after IV administration in 35 hospitalized adult horses on days 1, 3, 5, and 10 of treatment. The mean apparent elimination rate constant beta was 0.53 +/- 0.10 h-1 on day 1 for horses with normal plasma creatinine concentration and 0.41 +/- 0.13 h-1 for horses with abnormally high plasma creatinine concentration. There was no significant difference between beta of the hospitalized horses and of 6 healthy horses treated with gentamicin, but total clearance for the hospitalized horses with normal plasma creatinine concentration was significantly greater than that of the healthy horses and the hospitalized horses with abnormally high plasma creatinine concentration. Gentamicin clearance was significantly correlated with plasma creatinine concentration for healthy horses and hospitalized horses, and beta was significantly correlated with plasma creatinine concentration for hospitalized horses. Twelve of the 35 hospitalized horses required an adjustment in gentamicin dosage. Ten required a reduction, 1 an increase, and 1 a change in dosage and interval with no net change in total daily dose. One horse of 35 (2.9%) developed signs compatible with nephrotoxicosis on day 14 of treatment, despite appropriate gentamicin concentrations on day 10.