Preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose (0.01-1 mg kg-1) response curves were constructed for the alpha 2 adrenoceptor antagonists yohimbine, WY 26392 and idazoxan. Low doses of these drugs caused increases in preganglionic sympathetic nerve activity, heart rate and mean blood pressure, and decreases in femoral arterial conductance. In contrast, at the highest dose, all three drugs caused falls in mean blood pressure, decreases in heart rate and no further increases in preganglionic sympathetic nerve activity. The decreases in heart rate appeared to be due to increases in vagal tone since they were reversed by atropine methonitrate. These results demonstrate that low doses of alpha 2 adrenoceptors antagonists increase central sympathetic tone and blood pressure and are consistent with the hypothesis that central alpha 2 adrenoceptors are under a tonic input which is inhibitory to central sympathetic tone.