1. Neurotransmitters (noradrenaline, NA; chicken II luteinizing hormone-releasing hormone, LHRH) and activators of G proteins (GTP-gamma-S and AlF3) partially inhibit calcium current in bullfrog sympathetic neruones. Activation of the remaining current is slowed and shifted to more positive voltages. 2. The N-type calcium current appears to be the type modulated, since approximately 90% of peak current is blocked by omega-conotoxin (omega CgTx) and modulation is not affected by nisoldipine. 3. Calcium current at relatively negative voltages (-30 to -50 mV) is resistant to transmitter modulation. The current at such voltages is also resistant to omega CgTx, suggesting that it results from a different type of calcium channel. 4. The omega CgTx-resistant current includes dihydropyridine (DHP)-sensitive and DHP-resistant components. The omega CgTx- and DHP-resistant current is inhibited by transmitter agonist, but the DHP-sensitive (L-type) current is not. 5. In cells dialysed with a low concentration of calcium buffer (0.1 mM-BAPTA), transmitters still inhibit N-current incompletely. However, L-current was partially inhibited (approximately 10%) by LHRH, NA and the muscarinic agonist oxotremorine-M (OXO-M).