A digitalized 75-year-old patient with postoperative renal failure demonstrated a progressive rise in serum digoxin concentration, peaking at 3.4 nmol X L-1 three days following discontinuance of the drug. This was accompanied by cardiac bradyarrhythmias. Although the serum digoxin concentration had already started to climb from a therapeutic level prior to the discontinuance of the drug, the unabated and substantial rise was consistent with a dramatic decrease in the apparent volume of distribution of digoxin accompanying acute renal failure. Serum digoxin levels were determined with fluorescence polarization immunoassay, which has an improved specificity when compared to the commonly used radioimmunoassays for digoxin.