The relative bioavailability of isosorbide-5-mononitrate from two different formulations (MONOCLAIR, ISMO) was investigated in a double-blind randomized cross-over study in 8 healthy human subjects. After single orale doses of 40 mg the plasma concentration and the urinary excretion of isosorbide-5-mononitrate were measured over 24 hours. The comparison of the results obtained over the whole experimental period showed a significantly higher plasma concentration with a higher renal excretion after MONOCLAIR, the latter changes being insignificant as compared with those after ISMO, the reference drug. Thus the two formulations are judged to be at least equivalent with regard to their drug bioavailability.