We assessed the role of circulating digitalis-like substance(s) on the blood pressure regulation in patients with essential hypertension, cardiac diseases, diabetes mellitus and renal diseases by measuring digoxin-like immunoreactivity (DLI). Plasma DLI concentrations tended to correlate with blood pressure in all patient groups. Plasma DLI correlated to plasma aldosterone concentration in patients with essential hypertension, which suggested close interrelationship between DLI and electrolytes metabolism with adrenal steroids. Serum immunoreactive insulin (IRI) levels significantly correlated with blood pressure. Because plasma DLI levels correlated with serum IRI, increased levels of insulin could have induced sodium retention leading to increased DLI levels. Digitalis-like substance, but not insulin, would have directly increased blood pressure in patients with abnormal glucose tolerance. Plasma DLI levels significantly correlated with the severity of renal insufficiency in patients with renal diseases. Plasma DLI highly correlated with amounts of plasma proteins, particularly with albumin, which would be due to the binding of DLI with albumin in plasma. Because the level of non-binding DLI is extremely low when assayed with a digoxin-radioimmunoassay, it was impossible to assess the level of a free-form of DLI, i.e., active DLI. That could be a reason why the correlation between the DLI and the other parameters was not highly significant. Collectively, these findings suggest that the DLI is one of the major determinants of blood pressure rises, regardless of any cause.