1. The potencies of a number of selective α 2 -adrenoceptor antagonists have been measured in preparations from four species. 2. In the rat vas deferens, the newer synthetic antagonists Wy 25309, Wy 26392, Wy 26703 and RX 781094 were more potent than the alkaloid yohimbine in blocking a clonidine-induced inhibition of an electrically evoked twitch response. 3. In the rabbit vas deferens yohimbine was substantially more potent than the synthetic antagonists in reversing the action of clonidine. 4. Yohimbine was also more potent than the synthetic antagonists in blocking the B-HT 933-induced contractile responses of the dog saphenous vein and preventing adrenaline-induced aggregation of human platelets. 5. Together with previously published data derived from tritium overflow studies on rabbit pulmonary arteries and displacement of [ 3 H]-rauwolscine binding from rat cerebral cortex and human platelets, the results suggest that the adrenoceptor currently labelled α 2 may not be a single entity.