1. Whole-cell calcium currents of bullfrog sympathetic neurones were partially inhibited by noradrenaline (NA), chicken-II-luteinizing hormone-releasing hormone (LHRH), muscarine, ATP, substance P, or intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate)(GTP-gamma-S) or aluminium fluoride. These agents had similar effects on the activation kinetics of calcium current. 2. The amplitude of the LHRH effect varied from cell to cell. This did not correlate with cell size or the time of whole-cell dialysis. 3. The response to LHRH desensitized rapidly. Desensitization to LHRH did not affect inhibition by NA, ATP or substance P. 4. The effects of LHRH and NA were partially additive. 5. Cells dialysed with GTP-gamma-S still responded to NA or LHRH. However, NA or LHRH inhibited a smaller fraction of the calcium current than usual, and second applications of the same transmitter to GTP-gamma-S-dialysed cells were ineffective. 6. In GTP-gamma-S-dialysed cells, application of LHRH occluded the response to NA, but LHRH was still effective after application of NA. 7. The effect of GTP-gamma-S decreased during prolonged dialysis. 8. The effect of NA was selectively reduced by intracellular dialysis with the A-protomer of pertussis toxin (PTX), or extracellular pretreatment with high concentrations of whole PTX at room temperature. These treatments had little or no effect on the action of LHRH or ATP. 9. It is concluded that multiple G proteins can produce identical changes in calcium channel gating. The adrenergic receptor preferentially couples to a PTX-sensitive G protein.