This study has used receptor autoradiography to characterize imidazoline binding sites (I-BS) in monoamine oxidase (MAO) A knockout and wild-type mice. A comparison between MAO-A and MAO-B, binding of the endogenous beta-carboline [(3)H]harmane, and I-BS, has been made using sections from brain and kidney. The loss of binding to MAO-A in the knockout animals was confirmed using the selective radioligand [(3)H]Ro41-1049, with labelling reduced to background levels. The binding of [(3)H]Ro19-6327 to MAO-B was unaffected, indicating no change in this isoform in response to the loss of MAO-A. A reduction in binding to the I(2)-BS, as labelled by both [(3)H]idazoxan and [(3)H]2-BFI (2-(2-benzofuranyl)-2-imidazoline), was seen in the MAO-A knockout animals in both brain and kidney sections, whereas binding to the I(1)-BS in kidney sections remained unchanged. The loss of I(2) binding was found to be regionally dependent and was positively correlated with the relative expression of MAO-A in specific regions in the wild-type animals. Using the MAO-A knockout mice it was also possible to demonstrate a non-MAO-A population of binding sites labelled by the putative I-BS endogenous ligand, harmane.