Summary: The 3H-ouabain binding site concentration in the human myocardium was determined by measuring vanadate facilitated binding of 3H-ouabain to necropsy specimens of the left ventricle. The 3H-ouabain binding to samples weighing 4-6 mg was specific and saturable and appeared to take place to only one population of high affinity binding sites. After death the 3H-ouabain binding capacity degraded relatively slowly. From 6 to 24 h after death a mean decrease of 11% was seen in five patients, being significant in only one. In 15 patients aged 64-86 years the concentration of 3H-ouabain binding sites measured 6 h after death varied from 223 to 577 pmol·g-1 wet weight with no obvious relation to age or sex. The mean (SEM) value (413(26) pmol·g-1 wet weight) was 1.7 times higher than that previously reported for human myocardium. The concentrations of 3H-ouabain and 3H-digoxin binding sites were identical, and an excess of unlabelled ouabain completely prevented the specific binding of 3H-digoxin. In necropsy specimens weighing 1-2 mg from the endomyocardium obtained using a biotome the 3H-ouabain binding site concentration was in the same range as that in the myocardium.These findings indicate that it is possible to determine the concentration of Na, K-pumps in the human myocardium by measuring the 3H-ouabain binding capacity of biopsy specimens obtained during heart catheterisation or of specimens obtained within the first 18 h after death. This finding may be of importance for studying conditions in which the Na, K-pump concentration is suspected of undergoing variation.