1 Desensitization of G,-coupled receptors, the P2-adrenoceptor for example, involves rapid and slower components but little is known regarding the existence of rapid desensitization of Gi-coupled 2 Following stimulation by adrenaline or neuropeptide Y Ca2+ levels returned towards baseline a few minutes after agonist addition and were refractory to a second agonist exposure demonstrating rapid desensitization. Cross-desensitization experiments with neuropeptide Y, adrenaline and moxonidine demonstrated the presence of homologous (both receptors) and heterologous desensitization (neuropeptide Y receptors only), and that the x2A-adrenoceptor desensitization was not specific for phenylethylamine (adrenaline) or imidazoline agonists (moxonidine). 3 The protein kinase C activator, phorbol ester, rapidly desensitized the hormonal Ca2+ responses and inhibitors of protein kinase C enhanced the hormonal responses inconsistently. The tyrosine kinase inhibitor, herbimycin, enhanced Ca2+ mobilization by adrenaline and neuropeptide Y, whereas the protein phosphatase inhibitor, okdadaic acid, did not affect Ca2+ mobilization or its desensitization. 4 In the absence of extracellular Ca2+ the endoplasmic reticulum Ca2+-ATPase inhibitor, thapsigargin, reduced hormone-stimulated Ca2+ elevations, demonstrating that mobilization occurs from a thapsigargin-sensitive pool in the endoplasmic reticulum. The inositol phosphate-independent Ca2' release modulator, ryanodine, significantly enhanced adrenaline-and neuropeptide Y-stimulated Ca2+ elevations. Blockade of the endoplasmic reticulum Ca2+-ATPase by thapsigargin in the presence of extracellular Ca2+ enhanced hormone-stimulated Ca2+ increases, demonstrating the importance of this enzyme for the termination of the Ca2+ signal. 5 It is concluded that adrenaline and neuropeptide Y-stimulated Ca2+ mobilization in HEL-cells occurs from a thapsigargin-and ryanodine-sensitive store in the endoplasmic reticulum and desensitizes rapidly; this appears to involve multiple mechanisms including protein kinases, possibly acting on receptors, and Ca2+ release and sequestration mechanisms.