Understanding the mechanisms regulating the contractility and relaxation of uterine smooth muscle has important therapeutic implications for the treatment of preterm or dysfunctional labours. Modest external alkalinisation has been shown to be a potent modulator of uterine contractility; the spontaneous contractile function of uteri from rats was altered in a gestational-dependent manner by raising extracellular pH (pH_e) from 7.4 to 8.0 (Taggart et al., 1997). One possible mechanism of action of altered pH_e is to affect uterine smooth muscle electrical excitability by targeting K⁺ channels. In this study, therefore, we wished to investigate the effect of external alkalinisation on spontaneous contractions of human myometrium. In addition, we examined if any effect of external alkalinisation was modified by blockers of Ca²⁺– or voltage-activated K⁺ channels (TEA or 4-aminopyridine (4-AP) respectively). Human myometrial tissues from non-pregnant (hysterectomy, 17 strips n= 9) and pregnant women (elective Caesarean delivery at term, 14 strips n = 8) were obtained following informed written consent, dissected and mounted for isometric force recording in an organ bath and equilibrated in physiological saline (pH_e7.4) until regular spontaneous contractions occurred (1-2 hours). Extracellular alkalinisation to pH_e8.0 reduced spontaneous contractions in 5/8 myometria from pregnant women. Overall, contractile frequency was significantly reduced to 0.43±0.05-fold of control (mean±sem, n=8; p<0.05, paired t-test). When a contraction did occur in pH_e8, the amplitude was similar to control (1.00±0.03-fold, n=8). In the continued presence of pH_e8, application of the K⁺channel blockers TEA (5mM) or 4-AP (1mM) reversed the inhibition of spontaneous contractions such that the amplitudes were 1.45±0.07-fold (pH_e8/TEA, n=3; p<0.05) and 1.79±0.66-fold (pH_e8/4-AP, n=3), and frequencies were 1.21±0.61-fold (pH_e8/TEA, n=3) and 1.04±0.10-fold (pH_e8/4-AP, n=3) of control (pH_e7.4). In myometria of non-pregnant women, raised pH_edid not significantly reduce spontaneous contractions. These data suggest that in human myometrium, like rat uteri, there is a change in the sensitivity of uterine contractions to alkalinisation of pH_e8 with pregnancy. Furthermore, as the inhibitory influence of pHe8 on spontaneous contractions of myometria from pregnant women can be reversed by K⁺ channel blockers, this implicates that changes in membrane potential underlie the mechanisms of action of external alkalinisation.