The densities of the adrenergic innervation of the internal carotid and middle cerebral arteries and their extracerebral branches have been determined using fluorescence histochemistry. The density of the nerve plexus on the internal carotid artery was greater than that of the middle cerebral artery. The density of the plexus on the middle cerebral artery decreased with increasing distance from its origin. The density and the peripheral extent of the nerve fibre plexus on the arterioles arising from the carotid artery were greater than those arising from the middle cerebral artery. On any arteriole the density of innervation decreased with increasing distance from its origin. The passive electrical properties of proximal and distal middle cerebral arteriolar segments were compared. Both proximal and distal arteriolar segments had similar resistances and time constants in the order of 100 M omega and 250 ms respectively. Small regenerative responses could be elicited in all proximal middle cerebral arteriolar segments but only in a proportion of corresponding distal segments. The addition of external tetraethylammonium ions (TEA) provided much larger regenerative responses. Action potentials in proximal middle cerebral arteriolar segments had larger peak amplitudes and faster rise times than those of corresponding distal segments. Distal carotid arteriolar segments had similar voltage-dependent excitability as proximal segments of middle cerebral arterioles but generated less inward current for a given voltage step. There was a direct correlation between the density of innervation and the voltage-dependent excitability of arteriolar smooth muscle cells. The possibility that the presence of nerves is correlated with the density of calcium channels is discussed.