Clonidine reduced both the pithed rat heart rate and blood pressure responses to low frequency sympathetic nerve stimulation. Marked shortening of the blood pressure response but not of the heart rate or of the responses to injected noradrenaline was found. This shortening effect was independent of the number of impulses in the train (4--120 impulses) and of the impulse frequency. It lasted for more than 1 hour and was found to affect only the late part of the pressor decay curve. Nerve stimulations at 2 Hz simulating bursts of different periodicity in the splanchnic nerves showed that the cumulation of the blood pressure between bursts was affected only when these were applied at intervals of more than 10 sec. The effect could not be influenced by antagonists to several known transmitters, by ligation of the renal arteries or by adrenalectomy and does not seem to be mediated by pre-or postjunctional alpha receptor stimulation.