The effects of oral and intravenous administration of atenolol were studied in healthy volunteers. The oral administration of aseries of single doses of atenolol reduced an exercise tachycardia. After a 200‐mg dose, the effect on an exercise tachycardia was maximal at 3 hr and declined linearly with time at a rate of approximately 10% per 24 hr. The peak plasma atenolol concentration occurred at 3 hr and thereafter declined exponentially with time with an elimination half‐life of 6.36 ± 0.55 hr: 43 ± 3.9% of the dose was excreted in the urine within 72 hr. There was a correlation between the reduction in an exercise tachycardia and the logarithm of the corresponding plasma concentration. The intravenous administration of atenolol reduced exercise tachycardia with a signi(icant correlation between effect and plasma concentration. After 50 mg intravenously, 100% of the dose was recoveredfrom the urine, ami the clearance was 97.3 ml/min. Comparison of AUCo→x after oral and intravenous administration of 50 mg showed the bioavailability to be 63% after oral drug. Repeated oral administration of atenolol 200 mg daily either as a single dose or in divided 12 hourly doses for 8 days maintained reduction of an exercise tachycardia of at least 24% during the period of drug administration. The plasma elimination half‐life, area under the plasma concentration‐time curve, and peak plasma concentration after 200 mg atenolol were not changed by chronic dosing for 8 days.