Twelve patients with coronary heart disease were entered into a single-centre double-blind randomised crossover study to compare the effects of 2 dosages of isosorbide dinitrate (ISDN) spray (1.25 and 2.5mg) on haemodynamic parameters and exercise capacity at rest and during and after exercise. After a 36-hour washout period, baseline haemodynamic parameters [pulmonary capillary pressure (PCP), pulmonary arterial pressure (PAP), heart rate and blood pressure] were recorded at rest, under volume load (legs raised), and during and after bicycle ergonometry that followed an increasing workload protocol until the maximum load tolerated by the individual patient was reached. Haemodynamic parameters and ECG were monitored. 10 minutes after PCP had returned to baseline levels, ISDN spray 1.25 or 2.5mg was administered to a buccal, sublingual or lingual site, and the process repeated starting with measurement of resting parameters. A third cycle of measurements began after a 3-hour interval when the patient received the other dosage of ISDN. Both dosages of ISDN spray were well tolerated and no patients withdrew from the study. Both dosages significantly reduced PCP values measured under ergonomic load. There was no statistically or clinically significant difference between the 2 dosages. At rest, PCP was unaffected by either dosage. In conclusion, in this short term study both dosages of ISDN spray improved haemodynamics to the same extent in comparison with initial (untreated) values. The dose-effect relationship could not be assessed because of the high interpatient variation.