1. Transient outward current was recorded in cultured frog melanotrophs with the whole-cell configuration of the patch-clamp technique. The ionic dependence, kinetics and pharmacological properties of the current were studied. The effects of the A1 adenosine receptor agonist R-N6-phenylisopropyl-adenosine (R-PIA) on this current were also investigated. 2. In tetrodotoxin- and cobalt-containing solution, depolarization from -120 mV elicited both transient and delayed outward currents. Pulses from -60 mV activated only a sustained late current. 3. 4-Aminopyridine (4 mM) reduced the transient outward current much more than the delayed outward current. In contrast, tetraethylammonium (10-20 mM) selectively reduced the delayed current. 4. Tail current measurements showed a positive shift in the reversal potential when external K+ concentration was increased, indicating that K+ was the predominant charge carrier. 5. Steady-state inactivation was complete at potentials positive to -10 mV and removed by hyperpolarization. 6. Inactivation of the transient current was slowed and accelerated in oxidizing and reducing conditions, respectively, confirming the involvement of an inactivating 'ball and chain' peptide. 7. R-PIA increased the transient current. The steady-state inactivation curve was shifted towards more positive potentials without changing the activation kinetics. Pretreatment with pertussis toxin (1 microgram ml-1) blocked the response to R-PIA. 8. It is concluded that frog melanotrophs possess an A-type current that is likely to play an important role in excitability. This current, which is directly modulated by A1 adenosine receptors through a Gi/G(o) protein, appears to be responsible for the inhibitory effects of adenosine on electrical activity.