1. Despite the fact that numerous studies provide evidence for the existence of an endogenous inhibitor of the cell membrane Na+-K+ pump in plasma, little is known about the relationships between this factor and the main haemodynamic parameters.

2. In order to shed some light on this, we attempted to correlate haemodynamic parameters, measured during heart catheterization in a group of 22 cardiopathic subjects, with plasma digitalis-like activity levels, determined by two different procedures.

3. The ability of plasma to inhibit a human renal Na+-K+-ATPase showed an inverse correlation with cardiac index and a direct correlation with peripheral resistance. Plasma cross-reactivity with digoxin antibodies correlated directly with left atrial pressure.

4. These results furnish confirmation of a number of theoretical assumptions which attribute to the digitalis-like factor the ability to modify the contractility of the cardiovascular system.

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