The binding of [3H]clonidine to brainstem membrane preparations was studied in an attempt to characterize imidazoline-sensitive, catecholamine-insensitive receptors. Human samples and samples from two animal species were used. [3H]Clonidine binding was always saturable, reversible and specific with a KD value of 6-7 nM. The Bmax values were 45.5 +/- 5.5, 145 +/- 34 and 65 +/- 33 fmol/mg protein in the whole rat medulla oblongata, the nucleus reticularis lateralis region of bovine and that of human, respectively. In the whole rat brainstem we could not demonstrate the presence of [3H]clonidine binding sites that were insensitive to catecholamines. In bovine and human nucleus reticularis lateralis (NRL) preparations, the amount of specifically bound labelled clonidine that was not displaced by an excess of (-)-norepinephrine was 25 and 100%, respectively. Substances that had a structure similar to that of clonidine were able to compete with [3H]clonidine binding within the human NRL. Cirazoline was the most potent to inhibit [3H]clonidine binding although yohimbine was also able to displace binding in the human NRL but with lower apparent affinity. Competition assays with idazoxan stereoisomers clearly showed that this binding was stereospecific. Therefore the human NRL region provides the first model of an homogenous population of imidazoline-preferring, non-alpha-adrenergic membrane receptors.