Bioelectrochemical potentials of the rabbit hypothalamic supraoptic, paraventricular, suprachiasmatic nuclei, and medial preoptic area changed in a similar way in fever induced with i.v. administration of leucocytic pyrogen/interleukin I in all the structures due, probably, to their compact localization. The dynamics of bioelectrochemical activity coincided with that of rectal temperature derivative. The alterations of bioelectrochemical activity seem to reflect hypothalamic metabolic changes closely associated with effector mechanisms of thermoregulation in fever rather than with the temperature itself. The differences in the reaction patterns of neurosecretory nuclei are compared with changes in their vasopressin-synthetizing function during fever.