Atipamezole (MPV-1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole), a potent alpha 2-adrenoceptor antagonist, was tritiated to high specific activity. We then compared [3H]atipamezole and [3H]rauwolscine as radioligands for alpha 2-adrenoceptors in rat cerebral cortex, neonatal rat lung, and human platelets. (-)-Noradrenaline and phentolamine were used to define specific alpha 2-adrenergic binding. Unlabelled atipamezole was used in a similar manner to define saturable, high-affinity non-adrenergic binding. [3H]Atipamezole binding to human platelets (Kd 1.3 nM) and rat brain membranes (Kd 0.5 nM) equilibrated rapidly and was displaced in the expected manner by alpha 2-adrenergic ligands. In contrast, [3H]atipamezole binding in neonatal rat lung membranes was only effectively inhibited by unlabelled atipamezole, and by high concentrations of idazoxan. The total density of binding sites for [3H]atipamezole was clearly in excess of the density of alpha 2-adrenoceptors in this tissue, as defined by [3H]rauwolscine binding. We conclude that [3H]atipamezole binds with high affinity to alpha 2-adrenoceptors in human platelets and rat cerebral cortex, and that the compound can be used to investigate alpha 2-adrenoceptor properties and drug actions in these tissues. In neonatal rat lung, [3H]atipamezole identified an additional population of binding sites, distinct from both classical alpha 2-adrenoceptors and idazoxan-defined imidazoline receptors. The pharmacological identity of these binding sites remains to be elucidated. This non-adrenergic component in the binding characteristics of [3H]atipamezole complicates its use as a general alpha 2-adrenoceptor radioligand.