The activity of a series of busprione analogs at recombinant and rat thoracic aorta alpha-1 adrenoceptors was investigated. Compound affinity for recombinant alpha-1A, alpha-1B and alpha-1D adrenoceptors from human and animal sources was determined by radioligand binding assays using membranes prepared from rat-1 fibroblasts expressing recombinant receptors with ( +/- )-[125l]iodo-4-hydroxyphenyl)-ethyl-aminomethyl-tetralone as the radioligand. Compound affinity and functional activity at rat aortic alpha-1 adrenoceptors were determined using endothelium denuded rings contracted with phenylephrine. BMY 7378 ¿8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]-ehtyl)-8-azaspiro [4,5]decane-7,9-dione dihydrochloride¿ and MDL 73005EF ¿8-[2-(1,4-benzodioxan-2-ylmethylamino) ethyl]-8-azaspiro[4,5]decane-7,9-dione hydrochloride¿ were found to have significant selectivity for the alpha-1D-subtype and were high affinity antagonists of the alpha-1 adrenoceptors in the rat aorta. Leverage plot analysis of affinities of the buspirone analogs and a series of structurally diverse alpha-1 antagonists for recombinant alpha-1 adrenoceptors and rat aorta alpha-1 adrenoceptors demonstrate that the alpha-1 adrenoceptors in the rat aorta are predominantly of the alpha-1D subtype.