Both [3H]clonidine and [3H]idazoxan bind to alpha 2 adrenoceptors. The former also labels imidazoline receptors, and the latter non-adrenergic idazoxan binding sites. In order to investigate whether the imidazoline receptors and non-adrenergic idazoxan binding sites are identical, we compared the binding characteristics of [3H]clonidine and [3H]idazoxan to these sites by radioligand binding experiments on ultra-thin slices and homogenates of human striatum. A good correlation was found between the effect of different ions on the binding characteristics of [3H]clonidine and [3H]idazoxan, and the affinities of most competing drugs. However, clonidine and rilmenidine displayed a 100- and 10-fold lower affinity, respectively, for the idazoxan binding sites than for the imidazoline receptors. Autoradiography with [3H]clonidine showed that high densities of imidazoline receptors were present in the striatum, pallidum, gyrus dentatus of the hippocampus, amygdala, and substantia nigra. Moderate densities were found throughout the cerebral cortex, thalamus and several brainstem nuclei including the nucleus olivarius inferior. Low densities were seen in the cerebellum, spinal cord and pituitary gland. As for the non-adrenergic sites labelled by [3H]idazoxan, the imidazoline receptors can be found in all major brain areas examined. However, there are some striking differences between the concentrations of imidazoline receptors and non-adrenergic idazoxan binding sites in certain brain regions. To reconcile distribution and pharmacologic data, we propose that imidazoline receptors and non-adrenergic idazoxan binding sites represent different proteins or protein complexes and that at least in the nucleus reticularis lateralis and the striatum, imidazoline receptors and non-adrenergic idazoxan binding sites may be physically associated. The regional distribution of alpha 2 adrenoceptors within the human CNS was determined by quantitative autoradiography with [3H]RX821002. The highest densities of alpha 2 adrenoceptors were found in the cerebral and cerebellar cortex, and certain regions in the medulla oblongata (floor of the IV ventricle, reticular formation, hypoglossal nucleus and nucleus olivarius inferior). No alpha 2 adrenoceptors were detected in the pituitary gland. There exists no relationship between the distribution pattern of imidazoline receptors and alpha 2 adrenoceptors, indicating that these binding sites are independent from each other.