3H-alpha-Acetyl-digitoxin was administered to 5 patients i.v. and 3 patients p.o. The half-life of label in the plasma was 8.5 +/- 1 (i.v.) and 8.8 +/- 1 (p.o.) days. 20.9 +/- 3.6% (i.v.) and 21.3 +/- 2.9% (p.o.) of the radioactive dose were excreted into the urine in 6 days. Two patients excreted within 18 days 14.3 and 16.1% of the given dose with the stool. After oral administration 22.3% of the orally administered 3H-activity were eliminated into the feces by one patient. 63% (i.v.) and 53% (p.o.) of the chlorofrom-extracted 3H-activity in the urine could be attributed to digitoxin by means of thin-layer chromatography. The volatile content of plasma radioactivity was 4.07 +/- 0.1% (i.v.) and 6.78 +/- 0.2% (p.o.). The protein binding of a 4%. Albumin solution was 83 +/- 0.54%, for plasma 80.8 +/- 2%.