In order to assess the development of tolerance we analyzed in a placebo-controlled study the effect of monotherapy with isosorbide-5-mononitrate (IS-5-MN) 60 mg in a controlled release formulation (Durules) once-a-day. The IS-5-MN was evaluated after the first dose and after once-a-day therapy for three days in 11 ambulatory patients (10 males, 1 female, aged 54 +/- 9 years) with stable exercise-induced silent myocardial ischaemia and significant coronary stenoses. The drug was given at 8 o'clock in the morning, and a bicycle ergometer exercise test was performed after 4 hours. The ST segment depression was evaluated by a computer-assisted system. Standing blood pressure decreased during all three periods of active treatment with IS-5-MN, (in comparison with placebo p < 0.001 and p < 0.01, p < 0.01 respectively). Heart rate did not change significantly. Compared with placebo baseline values, ischaemic threshold increased during the first day of treatment (188 sec, p < 0.0001 at 4 hours), and to a lesser extent both in second (103 sec, p < 0.003) and third day (116 sec, p < 0.003). The total exercise time increased during all three days of active therapy but significantly so only during the first day. The exercise stress test performed in the 5th day during placebo demonstrated a high reproducibility of ischaemic-threshold (235 vs 241 sec, p: ns), implying that the improvement during the active treatment with IS-5-MN was not due to a "training effect". Headache in 2 patients was the only significant side-effect.(ABSTRACT TRUNCATED AT 250 WORDS)