A specific binding site for [3H]metergoline characterized by a KD of 0.5-1.0 nM was detected in microsomal and synaptic plasma membranes from various areas of the adult rat brain. Experiments with 5,7-dihydroxytryptamine- and kainic acid-induced lesions indicated that this specific binding site was localized post-synaptically with respect to serotoninergic neurons. The pharmacological characteristics of [3H]metergoline binding to microsomal membranes from the whole forebrain strongly suggest that this ligand labels a class of serotonin receptors. This was particularly obvious in the hippocampus in which serotonin was about 400 times more potent than dopamine and noradrenaline for displacing bound [3H]metergoline. In the striatum, serotonin was only 10 times as potent as dopamine in inhibiting [3H]metergoline binding, suggesting that this ligand may also bind to dopamine receptors. Striking similarities between the binding sites for [3H]metergoline and [3H]serotonin were observed in the hippocampus. Thus, not only the total numbers of binding sites for these two ligands in control rats but also their respective increases following intracerebral 5,7-dihydroxytryptamine treatment were very similar. Therefore, at least in the hippocampus, [3H]metergoline might well be the appropriate ligand for studying the characteristics of the 'antagonist form' of serotonin receptors postulated by Bennett and Snyder.