The technique of microelectrophoresis was used in order to examine the effects of clonidine on single neurones in the somatosensory cortex of the rat, and to compare its actions with those of noradrenaline and phenylephrine. Clonidine evoked only excitatory responses on cortical neurones. The clonidine-sensitive neurones were also excited by noradrenaline and phenylephrine. Clonidine had a consistently lower apparent potency than either noradrenaline or phenylephrine. Responses to clonidine had a slower time-course than responses to the other two adrenoceptor agonists, both the latencies to onset and the recovery times being longer for responses to clonidine than for responses to noradrenaline and phenylephrine. When the mobilities of clonidine and phenylephrine were compared using an in vitro method, no significant difference was found between the mobilities of the two ionic species, suggesting that they have similar transport numbers. Thus the difference between the potencies and time-courses of responses to clonidine and phenylephrine are presumably of biological origin. Responses to clonidine were antagonised by microelectrophoretically applied prazosin; responses to phenylephrine were equally antagonised, while responses to acetylcholine were not affected. Clonidine could reversibly antagonise excitatory responses to both noradrenaline and phenylephrine, without affecting responses to acetylcholine. The results suggest that clonidine may act as a partial agonist at excitatory alpha 1-adrenoceptors on cortical neurones.