1. The effects of noradrenaline (NA) on action potential shape and underlying membrane currents were examined in single smooth muscle cells freshly isolated from the ureter of the guinea-pig. 2. The voltage-dependent Ca2+ current (ICa) elicited upon depolarization from -50 to 0 mV was reduced by 27% upon application of 10 microM NA. This reduction was inhibited or converted to potentiation by internal application of low molecular weight heparin or 5 mM EGTA, indicating that it may be mediated by Ca(2+)-dependent Ca2+ channel inactivation via inositol 1,4,5-trisphosphate production and subsequent Ca2+ release from intracellular Ca2+ storage sites. 3. In contrast, Ba2+ current (IBa) through Ca2+ channels was potentiated by 36% in the presence of 10 microM NA. Internal application of GTP gamma S made it difficult to remove potentiation of IBa by wash-out; internal application of GDP beta S abolished potentiation. 4. NA caused a greater reduction in the transient Ca(2+)-dependent K+ current (IK(Ca)) upon depolarization than it did in ICa. This reduction was inhibited by internally applied heparin, suggesting that the amount of releasable Ca2+ in the storage sites was markedly reduced in the presence of NA. The sustained component of IK(Ca) which gradually increased during depolarization was also reduced by NA. 5. Action potential duration, which was recorded in a standard solution containing Ca2+, was prolonged by the application of NA. 6. It can be concluded that Ca2+ channel activity in ureter smooth muscle cells is regulated by a dual mechanism: Ca(2+)-dependent inhibition and GTP-binding protein-mediated potentiation. Under physiological conditions, both ICa and IK(Ca) were reduced by NA but the reduction of IK(Ca) was much larger than that of ICa; this results in an increase in net inward current during the action potential plateau and prolongs the action potential.