OR-1896 is a pharmacologically active, long-lived metabolite of levosimendan. In the current study, the metabolism of (14)C-labelled OR-1896 was investigated in six healthy men after intravenous infusion over 10 min and in male rats after an intravenous bolus dose. In human plasma, the only (14)C-compounds detected were (14)C-OR-1896 and its deacetylated form, (14)C-OR-1855, in varying proportions in different subjects. In rat plasma >93% of radioactivity was associated with OR-1896. Radioactivity was mainly excreted to urine in both rats (about 69% of the dose) and humans (about 87% of the dose). OR-1896 was a major urinary compound in both humans and rats. Another major human metabolite was hypothesized as N-conjugated OR-1855. Other human and rat urinary biotransformation products were characterized as N-hydroxylated OR-1896 and N-hydroxylated OR-1855, as well as glucuronide or sulphate conjugates of N-hydroxyl OR-1896. The main difference between rat and human metabolism was a lower amount of OR-1855-related metabolites in the rats. In human faecal homogenates, only OR-1896 and OR-1855 were detected, whereas rat faecal metabolite profile was similar to that in urine.