Single dose pharmacokinetics of atenolol (AT) enantiomers was studied in human volunteers and in rats. After oral administration of 50mg of racemic AT to humans, the areas under the plasma concentration–time curves (AUCs; mean ± SD) were 1640 ± 602 and 1860 ± 652 (ng/mL)h for the S(−)- and R(+)-enantiomers, respectively (p < 0.05). The small difference in the AUC was a reflection of a slight, but statistically significant (p < 0.05) difference in the renal clearance (CLr, mL/min) of the enantiomers [129 ± 32, S(−)-AT; 120 ± 29, R(+)-AT]. However, the two enantiomers were not different from each other (p > 0.05) with respect to the volume of distribution (Vλ, L/kg) [0.879 ± 0.342, S(−)-AT; 0.790 ± 0.255, R(+)-AT] or the terminal elimination rate constant (λz, h−1) [0.113 ± 0.038, S(−)-AT; 0.114 ± 0.036, R(+)-AT]. After iv administration of 10 mg/kg of the racemic AT to rats, the R(+)-enantiomer achieved higher AUC values [(ng/mL)h] compared with its antipode (p < 0.05) [3630 ± 1040, S(−)-AT; 4020 ± 1080, R(+)-AT]. Similar to the human results, this difference was due to a stereoselective renal clearance (mL/min/kg) in favor of S(−)-AT [14.9 ± 5.78, S(−)-AT; 13.0 ± 4.88, R(+)-AT; p < 0.05]. In rats, the Vλ (L/kg) of S(−)-AT was larger (p < 0.05) than that of its antipode (5.16 ± 1.83 versus 4.45 ± 1.11). The λz (h−1), however, was the same (p > 0.05) for both enantiomers [0.311 ± 0.106, S(−)-AT; 0.307 ± 0.079, R(+)-AT]. It is concluded that (a) the observed stereoselectivity in the pharmacokinetic parameters of the AT enantiomers in both humans and rats is negligible; (b) the underlying mechanism for stereoselectivity in the pharmacokinetics of the AT enantiomers in both species appears to be stereoselectivity in their renal clearances; and (c) the direction and extent of differences in the pharmacokinetic parameters of the enantiomers are the same in humans and rats.