Gentamicin sulfate, equivalent to 4 mg of gentamicin base/kg of body weight, was administered IV to 6 Thoroughbred foals on day 1 (12 to 24 hours of age) and at 5, 10, 15, and 30 days after birth. On day 40 after parturition, gentamicin was given to the mares at a dosage similar to that used in foals. Decay of serum gentamicin concentrations was best described by a 2-compartment model. Among foals, the overall elimination rate constant at 30 days of age was significantly (P less than 0.05) greater than at days 1, 10, and 15. There was, however, no difference in the overall elimination rate constant between foals and mares. The volume of distribution (Vd), determined on the basis of total area under the disposition curve, did not change between day 1 and day 30. Mean values of Vd of foals were between 1.5 and 2.5 times higher than the mean Vd of the mares; however, only values from the foals at days 5 and 10 were significantly greater. Both age and interindividual differences were reflected in the total body clearance (ClB) of gentamicin. Total body clearance of gentamicin of foals on day 1 was less than that of foals on days 5, 10, and 30. Additionally, ClB of gentamicin on day 15 was less than that on day 30. There was no significant difference between ClB of foals and mares except for the day-30 group, which had a higher clearance rate than did the adults. Protein binding of gentamicin was less than 30% in all groups, and there were no apparent age-related differences.