The activation kinetics of the inward-rectifier K+ channel were studied by single-channel recording in isolated single cells of the guinea-pig ventricle with two different extracellular concentrations of K+ ([K+]o 150 and 75 mM). When voltage pulses were applied from a holding potential more positive than the potassium equilibrium potential (EK), to potentials more negative than EK, the probability of the channel being in the open state (Po) increased with time after the onset of the command pulse. The ensemble averaged current increased in its initial phase (activation). When the command potential was more negative than EK-40 mV, the current decreased after rapid activation due to the inactivation of the channel. The averaged current could be divided into an instantaneous and a time-dependent activation component; the latter was fitted by a single exponential function. The time constant of the time-dependent component became shorter, at more negative command potentials. When compared at the same command potential, the instantaneous component became smaller, as the patch membrane was held at more depolarized potential. This indicates that the steady-state Po of the channel decreases with depolarization at potentials more positive than EK. The Po of the activation gate of the channel was estimated by dividing the steady-state Po of the channel by the Po of the inactivation gate at each potential. It was about 0.1 at EK + 20 mV and increased sigmoidally with hyperpolarization. At potentials more negative than EK-40 mV, the Po of the activation gate saturated and was almost 1. The single-channel analysis and the noise analysis of the steady-state current fluctuations revealed that the activation gate of the channel follows first-order kinetics between the open and closed states. The activation kinetics shifted along the voltage axis in a similar way as EK when different [K+]o were used. Thus, the activation of the channel depends not only on the membrane potential but also on EK, when [K+]o is changed. The macroscopic current flowing through the inward-rectifier K+ channel during the activation process was calculated, assuming that the elementary conductance of the channel is not voltage dependent. The calculated current showed a prominent inward-rectifying property in the steady state and formed a negative conductance region at potentials positive to EK. It was, therefore, concluded that the properties of the inward-rectifier time-independent background K+ current (iK1) in the whole-cell current records (Noble, 1979) mainly depend on the activation kinetics of the inward-rectifier K+ channel in the cardiac myocyte membrane.