The acute and chronic efficacy of slow-release isosorbide-5-mononitrate (IS-5-MN) in the form of Elantan long capsules (50 mg) and the tolerability to this agent were evaluated in an open study of 45 patients with chronic stable angina pectoris, treated for one year. After 3 days of replacement of previous antianginal treatment with placebo, an Elantan long capsule (50 mg) was given once daily in the morning. Exercise test was performed 6 hours after ingestion of the capsule, on placebo (after wash-out of 3 days), in the first day of treatment, and after 3 and 6 months of treatment as well as 6 and 24 hours after 12 months of treatment compared to the exercise test at the same time after discontinuation of treatment (three days on placebo). At comparable work load the drug was associated with a 26.6% reduction in ST-segment depression after 6 hours of acute treatment, 46.7% after 3 months of treatment, 52.2% after 6 months, and 66% at the end of treatment (12 months) (P less than 0.001). Even 24 hours after intake of the last capsule of Elantan long the ST-segment reduction was still 49.5%. Exercise test 24 hours after ingestion of the drug revealed that the effect still exists. In the post-treatment placebo period all values returned to pretreatment levels. The sublingual nitroglycerin consumption was reduced by 90% and the frequency of anginal attacks was reduced from 19 +/- 15 to 1.1 +/- 4.2 weekly (P less than 0.001). The drug was well tolerated.(ABSTRACT TRUNCATED AT 250 WORDS)