In a double-blind, randomized cross-over study the antianginal efficacy and tolerability of Elantan® Long (50 mg sustained-release IS-5-MN) were compared with a 80-mg sustained-release (s.r.) IS-5-MN formulation. 28 patients were enrolled in the study, which consisted of a 3-day washout period on placebo, an active treatment of 14 days (crossover after 7 days) and a final 3-day withdrawal period on placebo. Ergometric exercise tests with simultaneous ECG monitoring were carried out 2 and 10 h post administration (p.a.) on the last days of each period. Under 50 mg s.r. IS-5-MN the mean ST- segment depression under comparable load was reduced by 67% 2 h p.a. and by 70% 10 h p.a. The reduction under the 80 mg IS-5-MN form was about 76% 2 h p.a. and 78% 10 h p.a. Maximum workload, working capacity, duration of exercise, time to angina pectoris and time to ST-segment depression > 0.1 mV were markedly increased under therapy with each dosage. Comparisons between the medications revealed significant differences only in time to ST-segment depression > 0.1 mV and in ST-segment depression under maximum workload at 10 h p.a. which were increased more by the higher dosage. The incidence of anginal attacks decreased by almost 80% in the patients under both treatments. Ten patients under each dosage were completely freed from attacks. Adverse drug reactions occurred in 14 patients, 10 times under 80 mg s.r. IS-5-MN, 5 times under Elantan Long and twice under placebo. In two cases discontinuation of the study was necessary due to headache after IS-5-MN. Plasma levels of IS-5-MN (median without drop-outs) under steady state conditions under Elantan Long were of 361 ng/ml and 401 mg/ml 2 and 10 h p.a., respectively. For the 80-mg form 506 ng/ml and 647 mg/ml, respectively, were measured.