1. The effects of local applications of prostaglandin E2 (PGE2) on the unit activity of fifty-one neurones in the organum vasculosum lamina terminalis (OVLT) region and fifty-eight neurones in the preoptic area (POA) were investigated in small tissue slices from the rat hypothalamus containing the OVLT and POA isolated from each other. 2. Of these, thirty OVLT and twenty-eight POA neurones were warm sensitive and increased their discharge rate in response to a rise in tissue temperature. One OVLT neurone and one POA neurone were cold sensitive and showed the opposite type of responses to changes in temperature. The thermosensitivity of these neurones was still observed in a Ca2+ free-high Mg2+ solution. 3. Perfusion with PGE2 in doses between 1 and 250 nM changed the discharge rate in forty-two of fifty-one OVLT neurones and in thirty-two of fifty-eight POA neurones in a dose-dependent manner. The responses to PGE2 were not lost during synaptic blockade. The threshold dose of PGE2 to alter the discharge rate of the OVLT neurones (4.8 +/- 1.1 (S.E.M.) nM, n = 16) was significantly lower than that of the POA neurones (40.9 +/- 12.2 nM, n = 16). 4. Fifteen of forty-two OVLT neurones exhibited the responses with a slower onset (latency 5-13 min) and a longer duration (20 min to 3 h), but such responses were observed in only one of thirty-two POA neurones. 5. The responses of OVLT and POA neurones to PGE2 (50-250 nM) were reversibly blocked by a concurrent application of AH6809, a prostanoid EP1 and/or a DP receptor antagonist. 6. While there was no clear correlation between the type of thermosensitivity and the type of response to PGE2 among the POA neurones, a significantly higher incidence of inhibitory response to PGE2 was found among the warm-sensitive neurones in the OVLT region. 7. The lower threshold responses to PGE2 and the higher incidence of PGE2 responsiveness among OVLT neurones are consistent with previous findings which showed that the highest density of PGE2 receptor binding and the highest pyrogenic sensitivity to microinjected PGE2 were observed in the OVLT region. The results provide further evidence for the critical involvement of the OVLT region in mediating the febrile responses to blood-borne endogenous pyrogen through the local release of PGE2.