1 The effect of three different potassium channel blockers (tetraethylammonium, TEA; 4-aminopyridine, 4-AP; and apamin) and of variations in the concentration of K+ and Ca2+ in the medium, have been studied on the responses of guinea-pig isolated atria to (-)-N6_ phenylisopropyladenosine (R-PIA), a stable adenosine Al-receptor agonist, and to carbachol, a muscarinic agonist. R-PIA and carbachol showed the same negative inotropic effects over a similar range of concentrations (3-300 pM), both in spontaneously beating and in electrically driven atria. 2 TEA (0.1 to 20 mM) and 4-AP (0.3 to 3mM), both antagonized the negative inotropic and chronotropic effects of carbachol in a concentration-dependent manner. In contrast, these compounds failed to inhibit the effects induced by R-PIA. Apamin, a specific blocker of a low conductance Ca2 +-activated K+ channel, was ineffective in accordance with the absence of these channels in atrial tissue. 3 TEA (0.1 to 20mM) inhibited the negative inotropic effect of carbachol, but not that of R-PIA, in atria paced and depolarized by a high K+ medium (22mM). In this preparation Na+ current is abolished and the contraction induced by noradrenaline and electrical stimulation is solely dependent on Ca2+ influx currents. 4 Stepwise addition of Ca2+ to a calcium-depleted perfusing medium of electrically driven atria, induced a positive inotropic effect which was inhibited by R-PIA. In contrast, carbachol had no effect. 5 In agreement with our previous study, the data suggest that R-PIA acts on isolated atria by inhibiting Ca2 + influx through L-channels.