The digoxin in tablets from 4 different manufacturers was measured and found to be the same. The disintegration times for these tablets were different, and this measurement did not correlate with the plasma levels in subiects who took them. The tablet with the slowest disintegration rate produced satisfactory plasma digoxin concentrations. Another tablet, with slow dissolution rate, failed consistently to produce a therapeutic plasma level. All packages containing digoxin tablets should be labeled with data on their dissolution characteristics, but none should be sold unless these data are consistent with biologic availability.