[Problems posed by the interpretation of peroxidase labelling of various neurons in Cyprinidae]

The origin of a centrifugal visual pathway in Cyprinids could not be demonstrated with the technique involving the labelling of cell bodies by retrograde transport of Horseradish Peroxidase. The hypothalamic labelling following intraocular injection of HRP is localized in neurosecretory structures which take up the enzyme that has passed into the circulatory system. Identical results were obtained following direct intracardiac injection. Thus extreme caution must be taken in attempting to interpret HRP results.