The pharmacokinetics of atenolol with and without the co-administration of chlorthalidone were studied in five hypertensive subjects. Concomitant administration of chlorthalidone appears to have little if any effect on the pharmacokinetics of atenolol during treatment for 7 days. The atenolol elimination half-lives were 6.7±1.1 and 6.3±0.9 h, respectively, with and without chlorthalidone. Two healthy volunteers also received a single 50 mg oral dose of chlorthalidone. Their blood profiles and pharmacokinetics were similar to those observed in hypertensive subjects, but a statistically significant difference (p<0.01) was="" found="" between="" the="" urinary="" excretion="" half-lives="" of="" chlorthalidone.="" this="" difference="" may="" be="" because="" chronic="" administration="" of="" the="" drug="" caused="" saturation="" of="" red="" cell="">