The human syncytiotrophoblast acts as the principal physiological barrier between foetal and maternal blood. In this epithelium, functions such as maintenance of membrane voltage, solute transport and cell volume regulation, involve chloride currents. A major contributor to these currents is a Maxi-chloride channel present in the apical syncytiotrophoblast membrane. Diverse studies have demonstrated the modulation of other chloride channels by annexins, proteins that bind to acidic phospholipid membranes in a calcium-dependent manner. Within the family of the annexins, one of the most abundant members in syncytiotrophoblast is annexin VI; although various functions have been attributed to annexin VI, its precise physiological rol in placenta has yet to be defined. The purpose of this study was to evaluate the effects of annexin VI on the biophysical properties of the Maxi-chloride channel present in apical syncytiotrophoblast membrane from human placenta. Using isolated membranes prepared from term placental tissue through a protocol that includes differential centrifugation, precipitation with magnesium chloride and discontinuous sucrose gradient, and reconstituted into giant liposomes suitable for Patch-clamp recording of single channels, we have previously characterised the apical membrane Maxi-chloride channel: multiple substates, major substate conductance of 240pS, voltage dependent open probability (being open between ±50mV and closing at more extremes potentials), permeability to diverse anions (including amino acids), direct inhibition by arachidonic acid, steroid hormones and DIDS. The presence of annexin VI was detected in both sections of placental villi tissue and purified apical plasma membrane, using immunohistochemistry and western blot analysis with polyclonal anti-annexin VI antibody. Addition of 5μg/ml anti-annexin VI antibody in the bath solution of electrophysiological patch-clamp experiments induced a decrease in both the total chloride current (39± 1.9% decrease; n=4) and the Maxi-chloride major substate single channel conductance (105±13 pS; n=4). The anti-annexin VI antibody also induced marked flattening of the open probability versus voltage curve, suggesting a loss of voltage dependence of open probability of the Maxi-chloride channel. The addition of a preimmune serum did not induce significative changes in the mentioned variables with respect to values in control conditions. Our results suggest that the endogenous annexin VI of human syncytiotrophoblast regulates the Maxi-chloride channel of apical membrane. The modulation of these channels by annexin may be of great importance in placental physiology and pathophysiology.