The pharmacokinetics and bioavailability of cefadroxil in the rat were examined after intravenous and oral administration at three doses (2.5, 10, and 15 mg). Cefadroxil disposition kinetics was clearly nonlinear, with an increase in plasma clearance as the dose increased (2.65 +/- 0.55, 3.17 +/- 0.48, and 3.82 +/- 0.39 ml/min for the three doses assayed). This phenomenon was attributed to a saturable renal tubular reabsorption of the antibiotic. After oral administration, the normalized Cmax was lower for the largest dose (4.6 +/- 0.7 micrograms/ml) than for the other two doses (5.5 +/- 0.5 and 5.4 +/- 0.7 micrograms/ml). Renal excretion of cefadroxil in the rat after intravenous and oral administration was investigated at two level doses (2.5 and 15 mg). No significant differences were found between doses or administration routes, and the mean percentage of dose recovered in the urine for the intravenous and oral routes was 80.7 +/- 6.1% and 76.4 +/- 3.7%, respectively. Cefadroxil bioavailability estimated from plasma levels or from the amounts of drug excreted in the urine was high and ranged from 90% to 100%.