Using brain slices the effect of prostaglandin E2 (PGE2) on neurones from different locations of the rat hypothalamus was analysed. PGE2 (150 ng), when injected into the perfusion chamber, influences all hypothalamic neurones studied. The pattern of firing rate changes after PGE2 is variable, but the depressive effect predominates - 72 % of neurones decrease their firing rate in long-term experiments. PGE2 also lowers the thermosensitivity of warm sensitive neurones and increases the thermosensitivity of temperature insensitive neurones.