The effect of chloroethylclonidine on nodrenaline-induced contractions of the rat aorta was studied. Chloroethylclonidine (1.5 × 10-5 M) shifted noradrenaline dose-response curve to the right approximately 5000-fold without depressing the maximum. The response to noradrenaline after chloroethylclonidine was not antagonized by phenoxybenzamine (10-7 M), prazosin (10-7 M), WB 4101 (10-7 M) nor yohimbine (10-5 M) and is therefore not mediated via α1-adrenoceptors. These results would suggest that there is a homogenous population of chloroethylclonidine-sensitive α1-adrenoceptors in the rat aorta and that chloroethylclonidine treatment reveals a non-α-adrenoceptor noradrenaline binding site in this tissue.