The effects of open heart surgery on serum digoxin concentration--time curves were investigated in 10 cardiac patients receiving 0.25 mg/day digoxin. Blood samples were obtained from the patients immediately before and 1, 2, 3, 5, 8, 16 and 24 h after digoxin administration, both before open-heart surgery and 7 days after surgery. Serum digoxin concentrations, determined by fluorescence polarization immuno-assay, significantly (P < 0.05) increased after surgery, as did the maximum serum concentrations and the areas under the concentration-time curves. After surgery there was a significant increase in the serum gamma-glutamyl transferase concentration and a significant reduction in the total protein concentration. A reduction of digoxin dose may be appropriate for patients who have undergone open-heart surgery.