The bioavailability of digoxin and beta-methyl-digoxin (BMD) was tested with a single dose on the grounds of peak serum concentration, tmax, area under the serum concentration-time curve and the cumulative 24 hour urinary excretion on one group of patients with liver disease (n = 20) and one with gastrointestinal disease (n = 10). Despite the smaller dose (0.5 mg BMD against 0.75 mg digoxin), peak serum concentration was significantly higher with BMD in both groups and was also reached earlier than with digoxin. The extent of absorption was also higher in both groups with BMD than with digoxin. A comparison of the results on hand with the results obtained in corresponding tests on healthy persons showed no significant differences with BMD. The excellent bioavailability of BMD was therefore also proved on patients with gastrointestinal diseases, whereas with digoxin the absorption in these patients was retarded, compared with healthy persons, but the extent of absorption was not reduced either. In gastrointestinal diseases with unknown conditions of absorption, the better bioavailability of BMD is probably of advantage, when compared with digoxin. According to medical literature, t/2 of BMD may be prolonged in liver diseases, so that in such cases the dose of BMD has to be adjusted or the use of digoxin is recommended.