In rat-1 fibroblasts stably expressing rat α1d-adrenoceptors, noradrenaline and PMA markedly decreased α1d-adrenoceptor function (noradrenaline-elicited increases in calcium in whole cells and [35S]guanosine 5′-[γ-thio]triphosphate binding in membranes), suggesting homologous and heterologous desensitizations. Photoaffinity labelling, Western blotting and immunoprecipitation identified α1d-adrenoceptors as a broad band of 70–80kDa. α1d-Adrenoceptors were phosphorylated in the basal state and noradrenaline and PMA increased it. The effect of noradrenaline was concentration-dependent (EC50 75nM), rapid (maximum at 1min) and transient. Phorbol ester-induced phosphorylation was concentration-dependent (EC50 25nM), slightly slower (maximum at 5min) and stable for at least 60min. Inhibitors of protein kinase C decreased the effect of phorbol esters but not that of noradrenaline. Evidence of cross-talk of α1d-adrenoceptors with receptors endogenously expressed in rat-1 fibroblasts was given by the ability of endothelin, lysophosphatidic acid and bradykinin to induce α1d-adrenoceptor phosphorylation. In summary, it is shown for the first time here that α1d-adrenoceptors are phosphoproteins and that receptor phosphorylation is increased by the natural ligand, noradrenaline, by direct activation of protein kinase C and via cross-talk with other receptors endogenously expressed in rat-1 fibroblasts. Receptor phosphorylation has functional repercussions.