The ionic currents in isolated single atrial cells of the guinea-pig heart were analysed by the patch-clamp technique applied to whole-cell recordings and the effects of acetylcholine (ACh) on the membrane potential and currents were studied. Resting and action potentials of single isolated cells were normal. Upstroke velocity was sensitive to tetrodotoxin. Action potential duration was slightly shorter than in multicellular preparations. Voltage-clamp experiments demonstrated the presence of a Ca2+ current (iCa), and inward rectifying and outward rectifying K+ currents. The Ca2+ current was abolished by 2 mM-Co2+ or 10(-6) M-D600, and the K+ currents were greatly reduced by intracellular application of Cs+ using the patch electrode and simultaneously superfusing the cell with 5 mM-Cs+ Tyrode solution. Acetylcholine shortened the action potential duration in a dose-dependent manner. The threshold dose of ACh was about 10(-9) M and the maximal effect was obtained with 10(-6) M. The resting membrane potential was hyperpolarized by 1-3 mV. ACh (10(-8)-10(-6) M) increased the K+ currents both on depolarization and on hyperpolarization in a dose-dependent manner in the presence of 10(-6) M-D600. The ACh-induced K+ outward current revealed a progressive deactivation ('relaxation'), with a time constant of 111 +/- 16 ms at around 0 mV. When the K+ outward currents were minimized with Cs+, the reduction of iCa by 10(-8) M-ACh was insignificant, and became 17 +/- 2.5, 26 +/- 3.5 and 26 +/- 5% of the control value in 10(-7), 10(-6) and 10(-5) M-ACh, respectively. The inactivation time course of iCa recorded from the Cs+-loaded cells was not affected by 10(-7) M-ACh. These results suggest that ACh activates predominantly a K+ conductance in the guinea-pig atrium. The superimposition of a relaxing K+ outward current on iCa may lead to over-estimation of the decrease in iCa.