Myocardial Na, K-ATPase concentration was quantified in 18 0-8-year-old human subjects by vanadate-facilitated 3H-ouabain binding to intact samples of the left ventricle of the heart obtained at autopsy. Within the first 6 months of life, the Na, K-ATPase concentration showed a rapid decrease. The mean value in the age range from birth to 6 months was 1.6 times the mean value obtained in the age range 6 months to 8 years. The mean values ± SEM were 1,076 ± 57 (n = 10) and 671 ± 28 (n = 8) (p < 0.001) pmol/g wet weight, respectively. The highest value [1,433 ± 56 pmoUg wet weight (n = 5)] was found in a 3-month-old child and the lowest value [545 ± 22 pmol/g wet weight (n = 5)] in an 8-year-old. Evaluation of 3H-ouabain-binding kinetics showed no agedependent variations. The total amount of Na, K-ATPase found in the heart was ±30 and 80 nmol within the first 3 years of life and at 8 years, respectively. The age-dependent change in myocardial Na, K-ATPase concentration can be ascribed to variation in the ratio between the amount of Na, K-ATPase and muscle mass during development. Since myocardial Na, K-ATPase is the receptor for cardiac glycosides, the present results may in part explain the clinical observation that cardiac glycoside sensitivity and toxicity change in young age.