evidence for excitatory 5ht2receptors on rat brainstem neurones 1 the technique of microiontophoresis was used to investigate the identity of the receptor mediating the excitatory effects of 5hydroxytryptamine 5ht upon neurones in the midline of the medullary brainstem of the rat in vivo 2 the 5ht1like receptor agonists 5carboxamidotryptamine 5ct and 8hydroxy2dinpropylamino tetralin 8ohdpat failed to excite the majority of neurones excited by 5ht the mobilities of 5ct and 8ohdpat when tested in vitro were found not to differ significantly from that of 5ht suggesting that the lack of effect of these agonists was not due to a lower rate of release from the microelectrodes 3 the excitatory responses to 5ht were attenuated by the 5ht 2receptor antagonists ketanserin and methysergide when applied microiontophoretically or administered intravenously 03 and 1 mg kg1 respectively excitatory responses to glutamate and noradrenaline were not reduced 4 the 5ht3receptor antagonist mdl 72222 failed to attenuate selectively the excitatory response to 5ht when applied either by microiontophoresis or administered intravenously 1 mg kg1 5 microiontophoretic application of the alpha 1adrenoceptor antagonist prazosin did not attenuate excitatory responses to either 5ht or noradrenaline intravenously administered prazosin 08 mg kg1 also failed to attenuate excitatory responses to 5ht but did block excitatory responses to noradrenaline 6 these results suggest that 5ht2receptors but not 5ht1like receptors 5ht3receptors or alpha 1adrenoceptors are involved in the excitatory response of midline medullary neurones to 5ht