1 Plasma levels of atenolol and metoprolol and their effects on exercise heart rate have been studied after oral administration of single doses of 100 mg of the two drugs in ordinary tablets alone and during concomittant cimetidine medication of 1 g per day. 2 Cimetidine caused no significant changes in the bioavailability of any of the two beta-adrenoceptor blockers and the rate of elimination of metoprolol was unaffected by the histamine H2-receptor blocker. A slight but significant increase in the elimination half-life from 6.5 +/- 0.6 to 7.9 +/- 0.6 (P less than 0.05) was noted for atenolol after pretreatment with cimetidine. The beta-adrenoceptor blocking effect was about the same for the used doses of atenolol and metoprolol and it was not changed during cimetidine therapy.