Abstract1The hypothalamo‐hypophysial neurosecretory system in the Red‐winged Blackbird (Agelaius phoeniceus) has been studied with special reference to changes during the period prior to fall migration.2In September the supraoptic and paraventricular cells were found to be considerably larger with substantial amounts of aldehyde fuchsin positive neurosecretory material (NSM) accumulating in the perikaryon. Large amounts of the NSM were found also in the anterior median eminence and pars nervosa.3Towards migration time (November), the cells in the hypothalamic nuclei contained very little NSM. A marked decline in the amount of NSM was also seen in the anterior median eminence and probably in the pars nervosa too.4The release of NSM toward migration time has been discussed as an important event in the sequence of events that control and regulate migratory activ