α-adrenoceptor blocking drugs, pressor responses to noradrenaline, and the ankle jerk in man.

1 Changes in pressor responses to infused noradrenaline and in triceps surae jerk amplitude have been measured after the i.v. administration of: chlorpromazine, thioridazine, prochlorperazine, dimethothiazine, indoramin, benzoctamine and clonidine. 2 Each of the drugs blocked pressor responses to noradrenaline and reduced triceps surae jerk amplitude, but little correlation was found between their potencies in the two systems. 3 The effects of the drugs on the triceps surae jerk are not incompatible with an action at a spinal α-adrenoceptor site, but other pharmacological factors may be involved, such as penetration of the drugs into the central nervous system, and differences in their uptake(1) blocking activity.