1. [3H]-clonidine binds to membranes prepared from guinea-pig kidney. 2. At 25 degrees C the binding is rapid and saturable. 3. Scatchard analysis of the binding data showed that the Kd for [3H]-clonidine binding in kidney membranes is 8.54 nM and the density of binding sites 12.5 pmol/g wet wt. tissue. 4. Hill plots of the binding data showed that there were no cooperative site interactions associated with binding. 5. [3H]-clonidine binding could be displaced by drugs, the most potent being drugs with a high affinity for the alpha-adrenoceptor. The neuroleptic drugs (+)-butaclamol, cis-clopenthixol and cis-flupenthixol at high concentration also displaced [3H]-clonidine binding. 6. Drugs acting as agonists or antagonists of beta-adrenoceptors, histamine receptors, acetylcholine receptors as well as prostaglandins E1, E2, F1alpha and F2alpha, angiotensin II, arginine vasopressin, naloxone, nalorphine and pargyline had little effect on binding. 7. It is likely that the binding site labelled by [3H]-clonidine in guinea-pig kidney membranes is an alpha-adrenoceptor similar in some pharmacological aspects to an alpha2-adrenoceptor.