Comparative autoradiography revealed that the imidazolines [3H]p-aminoclonidine and [3H]idazoxan labelled high densities of alpha 2A-adrenoceptor sites in the inner medulla and inner stripe of the outer medulla of rat kidney. In contrast, the oxazoline [3H]rilmenidine labelled a high density of non-adrenergic sites in the cortex and outer stripe of the outer medulla, a lower density in the inner stripe and inner medulla and a low density of alpha 2A-adrenoceptor sites in inner medulla. The existence of novel, non-adrenergic oxazoline sites of potential functional importance in rat kidney has important implications for the classification of imidazoline receptors.