The effect of subarachnoid hemorrhage (SAH) on the adrenergic innervation of cat cerebral arteries was analyzed. Intracisternal injections of autologous blood reduced the noradrenaline content from the perivascular nerves. Dopamine beta-hydroxylase and the uptake of 3H-noradrenaline were also decreased. These changes returned to normal in a period of 2 to 3 weeks after the SAH. Superior cervical ganglionectomy and intracisternal injections of 6-hydroxydopamine also reduced these three parameters. It is suggested that SAH induces a transient adrenergic denervation of the cerebral arteries that might be involved in the production of the chronic cerebral vasospasm.