Introduction: The primary visual cortex of the primate shows distinct regions of increased cytochrome oxidase activity, the so called blobs. When visualized in tangential sections the blobs form a pattern of regularly distributed patches which are most prominent in the third cortical layer. Since cytochrome oxidase is an enzyme of the oxidative chain, increased local enzyme activity indicates an increased metabolic activity of a given region. It has been shown in the squirrel monkey cortex that this metabolic difference is reflected in the density of the cortical vascularization. The aim of this study was to replicate and extend this finding with a quantitative and layer specific assessment of the vascular density in blob and interblob regions. Methods: Formalin-fixed frozen sections of 2 animals (M. mulatta) were first stained with the classical cytochrome oxidase staining method that uses the enzyme activity to precipitate DAB. The sections were then further processed for fluorescence immunohistochemistry. They were incubated with anti-collagen type IV and a Cy3-conjugated secondary antibody to stain for blood vessels. Epifluorescence micrographs were taken and the vessels were manually and automatically delineated. The length density (mm/mm?) and volume fraction (mm?/mm?) of vessels was taken as a measure for the vascular density. The blobs were determined in the corresponding brightfield micrograph of the same section on the basis of the cytochrome oxidase stain. Results: The vascular lengt