Ischaemic-reperfusion cycles, and concomitant hypoxia-reoxygenation periods, are likely to generate reactive oxygen species (ROS). In human myometrium, such cycles likely occur during labour. An additional source of ROS may be the invasion of leukocytes into the human myometrium at the time of spontaneous labour (1,2). These raise the possibility that ROS generation within the myometrium is a mechanism governing labour-associated contractility. In many cell types, ROS have been associated with alterations in cellular excitability via effects on K⁺ channels (3). As myometrial excitability, and therefore contractility, is likely to be governed by K⁺ channels we examined the effect of hydrogen peroxide (H₂O₂) on whole-cell currents of uterine myocytes enzymatically isolated from biopsies collected from women at Caesarean section (following written informed consent). Whole-cell voltage clamp (using physiological K⁺ solutions) was used to assess the effects of H₂O₂ (10 or 100 μM) on transmembrane currents. Cells were held at -50mV and pulsed from -60 to +50mV in 10mV steps each of 200msec duration. All data were normalised to control current values and expressed as mean±SEM. Statistical comparisons of steady-state current levels (140 ms into a 200 ms sweep) between treatments were made at +40 mV, using arcsine transformation of individual data points followed by a paired t test. Peak outward current was 1.34±0.38 nA (n=16). Treatment of uterine myocytes with 10 μM or 100 μM H₂O₂ resulted in inhibition of outward current at all potentials positive to 0mV. The slopes of log transformed current-voltage plots were similar in control and H₂O₂ conditions. 10 μM and 100 μM H₂O₂ inhibited current at +40mV by 19.9±7.6% and 32.1±11.9% respectively (n=4; p<0.01 for both). Separate application of 4-aminopyridine (4AP, 5 mM), a blocker of voltage-dependent K⁺ currents, also resulted in reduction of outward current at positive voltages. At +40mV outward current was significantly inhibited by 36.6±7.9% (n=8, p<0.05). In a separate series of experiments, treatment with 4AP alone followed by 4AP and 100 μM H₂O₂ together gave current inhibitions of 25.9±5.7 % (n=5) and 23.6±5.7 % (n=5, p>0.05 compared to 4AP alone) respectively. We conclude that H₂O₂ inhibits K⁺ channel current sensitive to 4AP in human uterine myocytes. A likely uterine target, therefore, for H₂O₂, and potentially other ROS, is voltage-gated K⁺ channels.