1. The effects of various α-receptor agonists and antagonists on pre- and post-synaptic α-receptors of human isolated digital arteries and metacarpal (or metatarsal) veins have been studied.

2. The relative potency of phenylephrine compared with noradrenaline on the postsynaptic receptors was significantly greater in arteries than in veins.

3. The rank order of potency of various α-agonists on postsynaptic receptors was the same in arteries and veins except that in veins adrenaline was more potent than clonidine, whereas the reverse was the case in arteries.

4. Phentolamine showed no selectivity in either antagonizing postsynaptic responses or enhancing stimulation-induced transmitter release by antagonism at presynaptic α-receptors.

5. Yohimbine potently, but not selectively, antagonized postsynaptic responses to noradrenaline and phenylephrine in both arteries and veins, and enhanced stimulation-induced transmitter release to a greater extent in arteries than in veins.

6. Prazosin reduced the maximum response to noradrenaline (but not phenylephrine) to a significantly greater extent in veins than in arteries, and in veins alone shifted the phenylephrine curve to a greater degree than it did the noradrenaline curve. Prazosin did not significantly increase stimulation-induced transmitter release in either arteries or veins.

7. Phenoxybenzamine produced large increases in stimulation-induced tritium outflow which were significantly greater in arteries than in veins (even in the presence of cocaine).

8. It is concluded firstly that the postsynaptic α-adrenoceptors of these tissues are not a homogeneous population and that differing proportions of these receptors are present in the digital arteries and the metacarpal veins, and secondly that the sensitivities of the presynaptic α-adrenoceptors in the two tissues may also be different.

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