Several studies have shown that I2 imidazoline binding sites are localized on monoamine oxidases. Recent results obtained after solubilization of rat brain membranes and analysis by size-exclusion chromatography suggested the existence of additional I2 imidazoline binding sites located on proteins distinct from monoamine oxidases. In order to define whether such binding sites are expressed in human and rabbit liver, we solubilized I2 imidazoline binding sites and monoamine oxidases and compared their elution profile by size-exclusion chromatography. I2 binding sites were labeled using [3H]idazoxan. Monoamine oxidases were identified by the measure of [14C]tyramine oxidation and Western blot analysis using an anti-MAO-A/MAO-B polyclonal antiserum. After solubilization of rabbit or human liver using 1% digitonin, 90% of [3H]idazoxan binding eluted in a major peak corresponding to a Mr of approximately 175000 Da. A minor peak, (Mr approximately equal to 100000 Da) representing 10% of the recovered [3H]idazoxan binding, was also observed. [14C]tyramine oxidation as well as immunoreactive bands corresponding to MAOs were exclusively detected in fractions containing [3H]idazoxan binding. These results show that solubilized I2 imidazoline binding sites distinct from monoamine oxidases are not detectable in rabbit and human liver.