The labelling of rat cerebral cortex alpha 2-adrenoceptors with [3H]-yohimbine ([3H]-YOH) was investigated. At 25 degrees C, binding equilibrium was reached in about 10 min and dissociation occurred with a half time of about 1 min. Saturation experiments gave an equilibrium KD value of 10.13 +/- 1.95 nM and a maximum number of sites of 254 +/- 22 fmol/mg protein. The [3H]-YOH binding sites exhibited alpha 2-adrenergic receptor specificity; the order of potency for the antagonists was rauwolscine greater than yohimbine much greater than prazosin greater than corynanthine. For the agonists, the order was: oxymetazoline greater than clonidine greater than (-)-adrenaline greater than (-)-noradrenaline much greater than (-)-phenylephrine. Agonists exhibited shallow curves in inhibiting [3H]-YOH binding, with pseudo-Hill coefficients (nH) of less than 1.0. These curves were shifted to lower overall affinity and steepened in the presence of 100 microM GTP. Antagonist competition curves were also shallow but GTP had no significant effect. Divalent cations at millimolar concentrations decreased the [3H]-YOH binding: IC50 values were about 6.0, 6.8 and 0.3 mM for Ca2+, Mg2+ and Mn2+ respectively. The maximal number of [3H]-YOH binding sites in the cortex was close to that labelled by the agonist [3H]-paraaminoclonidine ([3H]-PAC). The regional distribution of these sites in the brain, examined at a single concentration of [3H]-YOH and [3H]-PAC, showed a similar pattern except in the striatum. Taken together, the results indicate that like [3H]-PAC, [3H]-YOH labels alpha 2-adrenoceptors in rat brain cortex. They also show that [3H]-YOH is a useful tool for the study of the high and low affinity sites.