The pharmacokinetics and dose proportionality of fexofenadine, a new non-sedating antihistamine, and its enantiomers were characterized after single and multiple-dose administration of its hydrochloride salt. A total of 24 healthy male volunteers (31 +/- 8 years) received oral doses of 20, 60, 120 and 240 mg fexofenadine HCl in a randomized, complete four-period cross-over design. Subjects received a single oral dose on day 1, and multiple oral doses every 12 h on day 3 through the morning on day 7. Treatments were separated by a 14-day washout period. Serial blood and urine samples were collected for up to 48 h following the first and last doses of fexofenadine HCl. Fexofenadine and its R(+) and S(-) enantiomers were analysed in plasma and urine by validated HPLC methods. Fexofenadine pharmacokinetics were linear across the 20-120 mg dose range, but a small disproportionate increase in area under the plasma concentration-time curve (AUC) (< 25%) was observed following the 240 mg dose. Single-dose pharmacokinetics of fexofenadine were predictive of steady-state pharmacokinetics. Urinary elimination of fexofenadine played a minor role (10%) in the disposition of this drug. A 63:37 steady-state ratio of R(+) and S(-) fexofenadine was observed in plasma. This ratio was essentially constant across time and dose. R(+) and S(-) fexofenadine were eliminated into urine in equal rates and quantities. All doses of fexofenadine HCl were well tolerated after single and multiple-dose administration.