1. The involvement of guanine nucleotide binding proteins in the coupling of D2 dopamine (DA) receptors to single potassium channels was examined in rat pituitary lactotrophs. 2. Lactotrophs were unambiguously identified by the reverse haemolytic plaque assay (RHPA) and membrane potentials, whole-cell and single channel currents recorded using patch electrode methods. 3. DA or the D2 selective agonist, quinpirole, induced the opening of single K+ channels in cell-attached patches underlying robust hyperpolarizations of membrane potential in single cells. 4. Both whole-cell and single channel responses were independent of Ca2+ or cAMP concentrations. 5. Pertussis toxin (PTX) pretreatment (50-250 ng/ml, 6-12 h) blocked the action of DA on lactotroph membrane potential and uncoupled D2 receptors from single K+ channels in cell-attached patches. 6. Internal dialysis with GDP beta S (guanosine 5'-O-(2-thiodiphosphate) greatly reduced whole-cell responses to DA in a dose-dependent manner. 7. Internal dialysis of lactotrophs with GTP gamma S (guanosine 5'-O-(3-thiotriphosphate) potentiated DA responses in a dose-dependent manner while rendering the responses irreversible at higher doses. 8. DA (100 nM) or quinpirole (10 microM) activated K+ channels in excised outside-out membrane patches that were identical to those identified in cell-attached patches in terms of conductance and gating kinetics. 9. It is proposed that D2 receptors are coupled to non-voltage-dependent K+ channels by G proteins of the Gi/Go class and that this coupling is via a direct, membrane delimited pathway.