The effects of 0.4 or 0.8 mg paraoxon/kg i.v. on circulation and regional pre- and postganglionic sympathetic activity were studied in anaesthetized rabbits. At both doses, the discharges of the preganglionic efferents increased slowly, whereas the changes in activity of postganglionic efferents differed. Concomitantly with a transient initial rise in blood pressure a temporary increase of discharges to the skeletal muscles and to the intestine was observed which is attributed to spontaneous firing of the regional ganglionic cells. After injection of 0.4 mg/kg there occasionally occurred a second pressor reaction conditioned by enhancement of preganglionic activity and facilitated transmission in certain sympathetic ganglia. After injection of the higher dose, postganglionic activity generally decreased except for the discharges of gastrointestinal efferents, which increased. Arterial blood pressure, cardiac output and total peripheral resistance fell markedly. A shock syndrome occurred which could be controlled by atropine injection if timely. From the results it was concluded that the higher dose of paraoxon blocked synaptic transmission in the paravertebral ganglia of the sympathetic trunk, and facilitated impulse transmission in the prevertebral abdominal ganglia.