Uterine smooth muscle cells (USMCs) undergo ionic channel remodelling during gestation to facilitate spontaneous action potential (AP)-driven contractions during labour. Electrical activity of USMCs is related to the intracellular Ca²⁺ dynamics which in turn controls their contraction. Simple models of [Ca²⁺]_i dynamics and mechanics of USMCs are available but a rigorous model for the membrane excitation is lacking. The aim of this study was to construct a mathematical description of an USMC at the late pregnant stage with biophysically detailed membrane electrophysiology. Mathematical models for thirteen ionic currents were developed based on voltage-clamp experimental data of late pregnant rat and human tissues in the literature. Several inward currents were considered: L-type Ca²⁺ current, attributed to be the major inward current, fast Na⁺ current, T-type Ca²⁺ current and an hyperpolarisation-activated current. The outward currents include: fast A-type transient K⁺ current, two voltage-gated K⁺ currents (I_K1 and I_K2), Ca²⁺-activated K⁺ current and a sustained background current. Ca²⁺-activated Cl^– current and a non-specific cation current, each have reversal potentials within the reported AP amplitude range, are also included. All currents are modelled as ohmic resistors and their conductance kinetics described by Hodgkin-Huxley-type first order ordinary differential equations. Currents of a Na⁺-Ca²⁺ exchanger and a Na⁺-K⁺ pump are also included to allow incorporation of [Ca²⁺]_i dynamics (via the membrane channels, the Na⁺-Ca²⁺ exchanger and the plasma membrane Ca²⁺-ATPase) modified from a simple uterine excitation-contraction model [1]. The model was initially validated by the ability to produce bursting APs by an external stimulus. In Figure 1(a), repetitive APs were evoked by a current clamp consistent with experimental recordings from myometrium of pregnant rats [2]. Further model parameterisation was evidenced by the ability to closely simulate the effects of oestradiol, which modulates the amplitude and activation properties of individual Ca²⁺ and K⁺ currents, to alter AP configuration to a tonic-like plateau [2-4]. In conclusion, a novel mathematical model of the electrical action potential of USMCs has been developed. This provides a powerful tool to investigate the ionic mechanisms underlying the physiological genesis of electrical excitation governing labouring contractions.