The patch clamp technique has been used to study ion channels in the undisturbed microvillous membrane of the human placental syncytiotrophoblast. In villi from 55 placentae delivered by caesarean section, high resistance seals were achieved in approximately 30 percent of attempts. Of these, a large conductance chloride channel was identified in seven inside-out and two 'cell' attached patches. The channel had the following properties: (a) a slope conductance of 313 +/- 9 pS, (b) the presence of sub-conductance states, (c) voltage dependency, being open predominantly between +/- 20 mV and inactivating at more extreme potentials and (d) inhibition by DIDS (4-acetamido-4'-diisothiocyanostilbene 2,2-disulphonic acid). These are characteristic features of 'maxi' chloride channels which have been identified in a variety of cell types (Gogelein, 1988). The role of the chloride channel in ion transport by or homeostasis of the syncytiotrophoblast has yet to be determined.