The characteristics of the alpha 2-adrenoceptor binding property of moxonidine (4-chloro-N-(4,5-dihydro-1H-imidazol-2-yl)-6-methoxy-2-methyl-5-pyrim idinamine, BDF 5895), a newly developed centrally acting antihypertensive agent, was investigated in receptor binding assays using [3H]-rauwolscine bound to rat cortex membranes. The results are as follows: 1. [3H]-Rauwolscine binds in a saturable manner to rat cerebral cortex membranes with a Bmax of 320 +/- 50 fmol/mg of protein and KD of 1.7 +/- 0.5 nmol/l. 2. The alpha 2-adrenoceptor antagonist yohimbine competes with a Ki value of 8.0 nmol/l and slope factor close to unity. (--)-Norepinephrine competes with an overall Ki-value of 50 nmol/l and slope factor of 0.61. Mg2+ concentrations of 10 mmol/l in the presence of 0.1 mmol/l 3'-guanylimidodiphosphate (Gpp(NH)p) (Mg2+/Gpp(NH)p) do not affect the yohimbine inhibition curve, but increase the (--)-norepinephrine Ki value to 2.8 mumol and the slope factor to unity. 3. Moxonidine competes with [3H]-rauwolscine binding with an overall Ki value of 460 nmol/l and slope factor of 0.72. Moxonidine binding inhibition was potentiated by Mg2+, but shifted to lower potency by Mg2+/Gpp(NH)p and 150 mmol/l NaCl. 4. Binding inhibition curves for (--)-norepinephrine and moxonidine fitted better to a 2-site model and could be interpreted in terms of high affinity site (with Ki value KH) and low affinity site (with Ki value KL). The effect of Mg2+/Gpp(NH)p and NaCl was to convert the high affinity sites to low affinity sites.(ABSTRACT TRUNCATED AT 250 WORDS)