Pharmacokinetic values of sodium amoxicillin (22 mg/kg of body weight) in foals were determined after a single IM injection in 6 Quarter Horse foals at 3, 10, and 30 days of age. Serum amoxicillin concentrations were measured serially over a 24-hour period. The absorption of amoxicillin was rapid and followed a 1st-order elimination. Mean peak serum concentrations occurred 30 minutes after the injection in foals at all ages and were 17.31 +/- 9.59 micrograms/ml when the foals were 3 days old, 23.28 +/- 9.86 micrograms/ml when the foals were 10 days old, and 21.35 +/- 6.39 micrograms/ml when the foals were 30 days old. Serum samples collected beyond 8 hours after administration contained amoxicillin concentrations less than 0.05 micrograms/ml. The elimination rate constant increased with increasing age (0.5265 +/- 0.0891 hour-1 when the foals were 3 days old, 0.6494 +/- 0.1114 hour-1 when the foals were 10 days old, and 0.7112 +/- 0.1016 hour-1 when the foals were 30 days old). Serum clearance increased with increasing age (498.4 +/- 82.6 ml/hr/kg at 3 days, 631.6 +/- 170.5 ml/hr/kg at 10 days, and 691.2 +/- 127.3 ml/hr/kg at 30 days). Serum half-life decreased with increasing age (1.34 +/-0.243 hour at 3 days, 1.10 +/- 0.239 hour at 10 days, and 0.991 +/- 0.139 hour at 30 days), whereas the extrapolated concentration at time zero and apparent volume of distribution did not change during the first 30 days of age.