The endogenous cytochrome oxidase activity of the pigeon's ectostriatum, the primary telencephalic structure of the tectofugal visual pathway, was histochemically demonstrated and a heterogeneous distribution of the reaction product was observed. In cross-sections the medial, central and ventrolateral parts of the ectostriatum showed high levels of activity while the centroventral and dorsolateral ectostriatum remained weakly labelled. Only slight left-right and interindividual variations were found in the pattern of labelling. These data demonstrate for the first time anatomical subdivisions within the ectostriatal core and open the possibility of functional parcellations within this structure.