The role of the N-terminus of human α 1 D -adrenoceptors was examined by deleting the first 79 amino acids (Δ 1 - 7 9 ) and epitope-tagging to facilitate immunoprecipitation and detection. Following transfection into HEK293 cells, 6- to 13-fold increases in the density of specific [ 1 2 5 I]BE 2254 binding sites were observed for both tagged and untagged Δ 1 - 7 9 α 1 D - compared to full-length α 1 D -adrenoceptors, while agonist and antagonist affinities remained unchanged. In contrast, immunoprecipitation of tagged receptors showed that full-length α 1 D -adrenoceptor protein was at least twice as abundant as Δ 1 - 7 9 α 1 D -adrenoceptor protein. Photoaffinity labelling with [ 1 2 5 I]arylazidoprazosin showed much more intense labelling of tagged Δ 1 - 7 9 α 1 D - than of full-length α 1 D -adrenoceptors. Substantial N-linked glycosylation of tagged Δ 1 - 7 9 α 1 D -adrenoceptors was observed, although full-length α 1 D -adrenoceptors contain two consensus glycosylation sites but are not glycosylated. These results suggest that N-terminal truncation of α 1 D -adrenoceptors enhances processing of a binding competent form in HEK293 cells; and show a clear dissociation between abundance of receptor protein and density of receptor binding sites.