The antipyretic effect of AVP, α-MSH and ACTH consists in lowering the thrmoregulatory threshold and in shortening the time span of the fever. Thus, neuropeptides influence activity of hypothalamic neurones regulating body temperature. This was confirmed by recent experiments of Moravec (this volume) which indicate that spontaneous activity and thermosensitivity of neurones in hypothalamic slices can be influenced, by AVP. Why neuropeptides of different chemical structure such as AVT, on one hand, and α-MSH and ACTH, on the other hand, induce the same effect on thermoregulation remains to be elucidated.