1. The effects of adenosine (50 microM) and 2-chloroadenosine (1-25 microM) were studied on Ca2+ currents in frog motor nerve endings. 2. Ca2+ currents associated with the synchronous, neurally evoked release of acetylcholine (ACh) were measured using either perineural or patch recording methods. Tetraethylammonium and/or 3,4-diaminopyridine were employed to block K+ currents. 3. Ca2+ currents were depressed by omega-conotoxin (1.5-2.5 microM), Cd2+ (100 microM-2 mM), Co2+ (500 microM-5 mM) or by a reduction of the extracellular calcium concentration. Such currents were also observed when Sr2+ was substituted for Ca2+. Both ACh release and Ca2+ currents at motor nerve endings have been reported to be insensitive to 1,4-dihydropyridine antagonists in this species. 4. Adenosine receptor agonists did not affect Ca2+ currents at concentrations that produced maximal inhibition of ACh release. 5. The effects of adenosine receptor agonists were examined on asynchronous K(+)-dependent ACh release under conditions in which the Ca2+ concentration gradient is likely to be reversed (Ca(2+)-free Ringer solution containing 1 mM EGTA). ACh release was measured by monitoring the frequency of occurrence of miniature endplate potentials (MEPPs). In Ca(2+)-free solutions containing 1 mM EGTA, high K+ depolarization caused a decrease in MEPP frequency, presumably because it elicits the efflux of Ca2+ from the nerve ending via membrane Ca2+ channels in a reverse Ca2+ gradient. 6. The Ca2+ channel blocker Co2+, which blocks the exit of Ca2+ from the nerve ending, increased the frequency of MEPPs in a concentration-dependent manner in a reverse Ca2+ gradient. 7. Adenosine or 2-chloroadenosine inhibited ACh release in a reverse Ca2+ gradient. 8. The results suggest that blockade of Ca2+ entry is not responsible for the inhibitory effects of adenosine at frog motor nerve endings.