The effect of old age on the net accumulation of 5-hydroxytryptamine (serotonin) into cerebrovascular nerves of rats was studied using a new, densitometric application of image analysis of immunohistochemical staining. The results showed evidence of reduced accumulation in old animals in nerves supplying arteries of the circle of Willis. Measurements of the density of nerves stained with an antibody to the neuronal marker, PGP9.5, showed that loss of nerve fibres may occur in some cerebral vessels in old age.