Zolertine is an alpha-adrenergic receptor antagonist that acts as an antihypertensive agent. Its effect was studied in animals in vivo and in vitro. Zolertine considerably decreased systemic blood pressure in mecamylamine hypertensive dogs in a dose-related fashion. Using the intravital microscopic method in rat's mesocygeus microvasculature a direct relationship between zolertine dosage and blockade was demonstrated as well as an inverse relationship between time of action of zolertine and percentage of vasoconstriction caused by noradrenaline. When only zolertine was applied, it caused a small vasoconstriction that decreased as its concentration increased which could be due to its ability to antagonize alpha receptor responses, but not beta responses. Zolertine is a more active alpha blocker than azapetin, a blocker used in medical practice. Competition binding experiments using the alpha1-adrenoceptor antagonist [3H] prazosin showed a zolertine pKi of 6.81 +/- 0.02 in rat liver (alpha1B-adrenoceptors) and 6.35 +/- 0.04 in rabbit liver (alpha1A-adrenoceptors) membranes. Zolertine showed higher affinity for alpha1D-adrenoceptors compared to alpha1A-adrenoceptors, while it had an intermediate affinity for alpha1B-adrenoceptors. The ability of the alpha1-adrenoceptor antagonist zolertine to block alpha1D-adrenoceptor-mediated constriction in different vessels of Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats may explain its antihypertensive efficacy despite its low order of potency.