Alpha-2 adrenergic receptors have been divided into four pharmacological subtypes based on their differences in affinity for several drugs. Previous studies showed that [3H]RX821002 has a high affinity for the alpha-2A subtype. The current study characterized the binding properties of [3H]RX821002 [2-(2-methoxy-1,4- benzodioxan-2yl)-2-imidazoline] to the alpha-2A receptor in CHO-C10 cells, alpha-2B in neonatal rat lung, alpha-2C in OK cells and alpha-2D in bovine pineal gland. Membrane binding studies of [3H]RX821002 were done in 25 mM glycylglycine buffer at room temperature. The nonspecific binding rates at the KD concentration were 4.9%, 20%, 14% and 8.3% of the total for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively, which were determined by adding 100 microM norepinephrine. Saturation curves indicate that [3H]RX821002 has a high affinity for all alpha-2 adrenergic subtypes. The KD values were 0.29, 1.05, 0.37 and 0.19 nM for CHO-C10, neonatal rat lung, OK cells and bovine pineal, respectively. [3H]Rauwolscine has affinities of 0.34, 0.55 and 0.24 nM for the alpha-2A, -2B and -2C subtypes. By contrast, [3H]rauwolscine has a much lower affinity for alpha-2D subtype with a KD value of 5.2 nM. The binding site density for [3H]RX821002 was significantly lower in the neonatal rat lung compared with [3H]rauwolscine. The correlation coefficients of pKi values of adrenergic compounds against [3H]RX821002 versus [3H]rauwolscine were close to unity for each tissue. These data clearly show that the two ligands label the same alpha-2 adrenergic receptor population.(ABSTRACT TRUNCATED AT 250 WORDS)