A double-blind crossover study, with placebo periods preceding each active treatment, was carried out in 29 patients with ischaemic heart disease to compare the clinical efficacy of isosorbide 5-mononitrate 40 mg twice daily and the same regimen of sustained-release isosorbide dinitrate. Assessment was by bicycle ergometry, symptomatology and blood nitrate level measurements. After 2-weeks' therapy with isosorbide 5-mononitrate, mean ST segment depression at the highest comparable exercise level for each patient was reduced by 32.5% (p less than 0.01), the mean maximum exercise level was increased from 87 to 102 Watts (17.8%, p less than 0.01), and mean work capacity was increased from 581 Watts-minutes to 783 Watts-minutes (34.6%, p less than 0.01). With sustained-release isosorbide dinitrate, ST segment depression was reduced by 16.5% (p less than 0.05); maximum exercise level showed only a small non-significant increase, from 89 to 97 Watts (8.6%), but work capacity increased (p less than 0.05) from 593 Watts-minutes to 705 Watts-minutes (18.9%). There was no significant difference between drugs as regards ST segment reduction but a slight difference in favour of isosorbide 5-mononitrate (p less than 0.05) in terms of maximal exercise level and work capacity. Both drugs resulted in very small reductions in blood pressure (p less than 0.001 for systolic with isosorbide 5-mononitrate; p less than 0.01 for systolic and p less than 0.05 for diastolic with sustained-release isosorbide dinitrate). Heart rate and rate-pressure product changed little with either drug.(ABSTRACT TRUNCATED AT 250 WORDS)