Six-barrelled glass micropipettes were used to record the extracellular activity of neurones in the rostromedial hypothalamus of methoxyflurane-anesthetized cats, and to apply prostaglandins (PG), noradrenaline, 5-hydroxytryptamine, and acetylcholine by microiontophoresis to the vicinity of each neurone encountered. One hundred and sixty-three neurones were tested in 31 cats for responsiveness to prostaglandins and to amines. Thirty-three of these neurones were identified by their responsiveness to peripheral thermal stimulation, which was applied by blowing warm (42 °C) and cold (4 °C) air in the faces of the animals. The remaining neurones were functionally unidentified. Thirty-six prostaglandin responses were evoked in 27 neurones. Ten percent of those tested responded to PGE1, 17% to PGE2, and 17% to PGF2α. The majority of these responses (69%) were fast (short latency, rapid termination) type. Slow onset and termination were typical of 25% of the responses. Of 21 neurones which were sensitive to PGE1 or PGE2 17 (77%) were thermoresponsive, 10 being cold excited and 7 warm excited. Five neurones were sensitive only to PGF2α, and all were nonthermoresponsive. Neurones sensitive to PGE1 or PGE2 were significantly more likely to respond to NA (p < 0.05) and 5HT (p < 0.01) than PGE-insensitive neurones. Amine responses were not altered by simultaneous application of PGE. The data support the view that PGE2 and PGF2α are more active in this region than PGE1, and that the action of either PGE1 or PGE2 is specific to thermoregulatory neurones in the rostral hypothalamus. Evidence has been obtained for a common site of action of PGE and the amines NA and 5HT on thermoregulatory neurones in the cat.