Rapid steroid actions have been widely recognized only recently, e.g. on intracellular electrolytes, e.g. free intracellular calcium in vascular smooth muscle cells as determined by fura 2 spectrofluorometry in single cultured cells from rat aorta. Latter effects are almost immediate and reach a plateau after only 3 to 5 minutes. This mechanism operates at physiological subnanomolar aldosterone concentrations and appears to result in rapid fine tuning of cardiovascular responsivity. As a landmark feature of many of these rapid effects, insensitivity to spironolactone has been found. These effects translate into biological actions e.g. in the vasculature of animals and humans. Both constriction or dilation can occur presumably depending on the extent of endothelial involvement. Even blood pressure is subject to rapid changes in response to aldosterone if counterregulatory effectors such as the adrenergic system are blocked. Aldosterone seems to rapidly affect peripheral resistance both through direct action at vascular smooth muscle cells and endothelium, and by effects on the baroceptor function. It even influences cardiac action potential duration, an effect which might be linked to its known pro-arrhythmic activity. There is a medical need for more complete antagonism of mineralocorticoid action than that achieved by spironolactone.