Neuropeptide Y (NPY) is a 36 amino acid peptide which was shown to enhance memory retention, recall and prevent amnesia induced by either scopolamine or anisomycin. In this study, we examined the effects of NPY administration into 6 areas of the mouse brain on memory retention for footshock avoidance training in a T-maze. NPY was injected into the rostral and caudal hippocampus, amygdala, caudate, septum and thalamus shortly after training. NPY improved retention when injected into the rostral portion of the hippocampus and septum, impaired retention in the caudal portion of the hippocampus and amygdala and had no effect in the thalamus and caudate. NPY was ineffective at either improving or impairing retention when injected 24 h after training, thus demonstrating that the effects of NPY on retention were time-dependent and not due to proactive effects on retention test performance per se. In addition, NPY had no effect on retention when injected into overlying cortical areas. NPY antibody impaired retention when administered into the rostral hippocampus and septum; it improved retention in the caudal hippocampus and amygdala. Thus NPY antibody had the opposite effect to that of NPY on memory retention suggesting that NPY has a physiological role as a modulator of memory processing within specific anatomical areas of the central nervous system.