The effects of adrenergic receptor ligands on uptake and storage of the radiopharmaceutical [125I]metaiodobenzylguanidine (MIBG) were studied in the human neuroblastoma cell line SK-N-SH. For uptake studies, cells were incubated for 15 min with varying concentrations of alpha-agonist (clonidine, methoxamine, and xylazine), alpha-antagonist (phentolamine, tolazoline, phenoxybenzamine, yohimbine, and prazosin), beta-antagonist (propranolol, atenolol), beta-agonist (isoprenaline and salbutamol), mixed alpha/beta antagonist (labetalol), or the neuronal blocking agent guanethidine, prior to the addition of [125I]MIBG (0.1 microM). The incubation was continued for 2 h and specific cell-associated radioactivity was measured. For the storage studies, cells were incubated with [125I]MIBG for 2 h, followed by replacement with fresh medium with or without drug (MIBG, clonidine, or yohimbine). Cell-associated radioactivity was measured at various times over the next 20 h. Propanolol reduced [125I]MIBG uptake by approximately 30% (P<0.01) at all concentrations tested, most likely due to nonspecific membrane changes. However, incubation with the other beta-agonists or antagonists failed to elicit significant reductions in uptake. In contrast, all of the alpha-agonists significantly inhibited uptake (P<0.05); guanethidine >xylazine >clonidine=methoxamine. The alpha-antagonists demonstrated a broad range of inhibition (phenoxybenzamine >>phentolamine >prazosin >>yohimbine=tolazoline)(P< 0.05). The mixed ligand, labetolol, inhibited MIBG uptake in a dose-dependent manner with an apparent IC50 of 0.65 microM. The retention studies demonstrated that unlabeled MIBG caused profound self-inhibition (P<0.01). Clonidine produced a modest inhibition of retention and yohimbine had no effect. Labetalol, phenoxybenzamine, guanethidine, and propranolol reduced uptake of [125I]MIBG by neuroblastoma cells in culture. Although only labetalol has been reported to cause false-negative MIBG scans, our results suggest that these other drugs have the potential to interfere with MIBG imaging and therapy, particularly at high doses. Adrenergic drugs did not alter cytoplasmic retention of [125I]MIBG in neuroblastoma cells but may have potential in tumors such as phenochromocytoma, where granular storage of MIBG has been observed. Inhibition of [125I]MIBG retention by unlabeled MIBG supports the use of high specific activity radioiodinated MIBG for both diagnosis and therapy.