Elementary K + currents were recorded at 19 °C in cell-attached and in inside-out patches excised from neonatal rat heart myocytes. An outwardly rectifying K + channel which prevented Na + ions from permeating could be detected in about 10% of the patches attaining (at 5 mmol/1 external K + and between -20 mV and + 20mV) a unitary conductance of 66_+ 3.9 pS. + K(o.t ...... ,.) channels have one open and at least two closed states. Open probability and Zop¢. rose steeply on shifting the membrane potential in the positive direction, thereby tending to saturate. Open probability (at -7 mV) was as low as 3 + 1% but increased several-fold on exposing the cytoplasmic surface to Mg-ATP (100 ~tmol/1) without a concomitant change of %po~. No channel activation occurred in response to ATP in the absence of cytoplasmic Mg + +. The cytoplasmic administration of the catalytic subunit of protein kinase A (120-150/2/ml) or GTP-7-S (100 #tool/l) caused a similar channel activation. GDP-fl-S (100 #tool/l) was also tested and found to be ineffective in this respect. This suggests that cardiac + K(o,t ...... ~.) channels are metabolically modulated by both cAMP-dependent phosphorylation and a G-protein.