1. Microinjection of selective antibodies into superior cervical ganglion (SCG) neurones has identified the G-protein alpha-subunits mediating muscarinic receptor inhibition of M-type K+ current (IK(M)) and alpha-adrenoceptor inhibition of Ca2+ current (ICa). 2. Antibodies specific for G alpha q/11, but not those for G alpha o, reduced M-current inhibition by the muscarinic agonist oxotremorine-M, whereas anti-G alpha o antibodies, but not anti-G alpha q/11 or anti-G alpha i1-3 antibodies, reduced calcium current inhibition by noradrenaline. 3. Immunoblots with specific anti-G-protein antibodies demonstrated the presence of both G alpha q and G alpha 11, while G alpha o1 (but virtually no G alpha o2) was present. 4. We conclude that M1 muscarinic receptor inhibition of IK(M) is transduced by G alpha q and/or G alpha 11, and that G alpha o transduces alpha-adrenoceptor inhibition of ICa.