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Alterations in the flow of blood to and from the penis are thought to be the most frequent causes of male erectile dysfunction and, therefore, the present review focuses on the penile vasculature. In the flaccid state, tonic noradrenaline release from the sympathetic nerves contracts penile arterial and corporal smooth muscle through activation of postjunctional α₁-adrenoceptors, both by increasing intracellular calcium and by enhancing the sensitivity of the contractile apparatus for calcium. In addition, noradrenaline inhibits vasodilatatory neurotransmitter release by prejunctional α₂-adrenoceptors. The exact role of the sympathetic neurotransmitters, neuropeptide Y and adenosine 5′-triphosphate, in erection is largely unknown. Penile vasodilatation during erection is mediated by nitric oxide (NO) through activation of guanylyl cyclase in the smooth muscle layer, followed by increases in cyclic guanosine …