The characteristics of [3H]RX821002 binding to the different human alpha 2-adrenoceptor subtypes were studied on membranes from COS-7 cells transfected with the genes: alpha 2C2, alpha 2C4 and alpha 2C10. Saturation experiments indicated that the radioligand labels the three adrenoceptors with high affinity. A difference was however observed between the subtypes. The affinity of [3H]RX821002 for alpha 2C10-adrenoceptors (KD = 1.41 +/- 0.15 nM) was 3-fold higher than for alpha 2C4-adrenoceptors (KD = 4.42 +/- 0.63 nM) and 7-fold higher than for alpha 2C2-adrenoceptors (KD = 10.2 +/- 0.9 nM). Inhibition experiments with a series of 17 competitors confirmed that prazosin, oxymetazoline, WB4101, ARC239, corynanthine and chlorpromazine are subtype-selective drugs. They also demonstrated that BRL44408 and guanfacine are selective for the alpha 2C10-receptor, whereas BRL41992 and imiloxan are selective for the alpha 2C2. Given that these two latter drugs were previously shown to be specific for the alpha 2B pharmacological subtype originally defined in neonatal rat lung, these results confirm that the alpha 2C2 gene encodes for the human homolog of this receptor subtype. It is concluded that the combined use of [3H]RX821002 and of these new selective drugs may be useful for the identification of the alpha 2-adrenoceptor subtypes in human tissues.