Marinobufagenin, the inhibitor of the Na+,K+ ATPase, belongs to the group of cardiotonic bufadienolide steroids. It was first discovered in the skin of the toad Bufo marinus, where its physiological function consists in the regulation of sodium excretion. In the 1990s, bufadienolide compounds were discovered in mammals. The concentrations of the endogenous digitalislike factors in the systemic circulation are usually very low, the concentration of ouabain varying in the range of 300–1000 pM and that of marinobufagenin being about 400 pM. Increased levels of endogenous marinobufagenin can be detected in a number of pathological states, such as essential hypertension, cardiac infarction, diabetes mellitus, and chronic renal dysfunction. It was determined that marinobufagenin contributes to the development of generalized vasoconstriction and gestational toxicosis.