Excised patches were used to study the kinetics of a Cl channel newly identified in cultured human fibroblasts (L132). The conductance of ca. 70 pS in 150 mm symmetrical Cl, and the marked outward rectification ascribe this channel to the ICOR family. Long single-channel recordings (>30 min) revealed that the channel spontaneously switches from a kinetic mode characterized by high voltage dependence (with activity increasing with depolarization; mode 1), into a second mode (mode 2) insensitive to voltage, and characterized by a high activity in the voltage range +/-120 mV. On patch excision the channel always appeared in mode 1, which was maintained for a variable time (5-20 min). In most instances the channels then switched into mode 2, and never were seen to switch back, in spite of the eight patches that cumulatively dwelled in this mode 2.33-fold as compared to mode 1. Stability plots of long recordings showed that the channel was kinetically stable in both modes, allowing standard analysis of steady-state kinetics to be performed. Open and closed time distributions of mode 1 and mode 2 revealed that the apparent number of kinetic states of the channel was the same in the two modes. The transition from mode 1 into mode 2 was not instantaneous, but required a variable time in the range 5-60 sec. During the transition the channel mean open time was intermediate between mode 1 and mode 2. The intermediate duration in the stability plot however is not to be interpreted as if the channel, during the transition, rapidly switches between mode 1 and mode 2, but represents a distinct kinetic feature of the transitional channel.