In five children with insufficiency of the heart the blood digitalis-digoxin level was monitored by drawing blood 5 times daily over a period of 7 days, starting from the first day of digitalisation. The dosage aimed at a mid-rate attainment of the plateau concentration in the blood. Digitalis blood levels were determined by a commercial radioimmunoassay. The blood level curves were simultaneously adjusted over the whole period, with the aid of a self-developed programme for tablecacultators, so that the considerable fluctuations of blood levels could be exhibited. The so adjusted curves were also pharmacokinetically analysed. Our data are in good agreement with the literature. The total clearance was significantly below that of adults. The total amount of digoxin in the whole organism, as calculated and represented graphically, changed less significantly, but was clearly higher in the first days of the mid-rate digitalisation than during the steady state.