Continual renewal of multinucleate syncytiotrophoblast is important for successful pregnancy. Renewal occurs by a process of cell turnover involving proliferation, differentiation and incorporation of cytotrophoblast cells, balanced by apoptotic shedding of syncytiotrophoblast nuclei. Trophoblast turnover is under autocrine/paracrine regulation by human chorionic gonadotropin (hCG), itself produced by differentiated syncytiotrophoblast. In many tissues, cell turnover and endocrine secretion are regulated by K⁺ channels (O’Grady and Lee, 2005) but the involvement of K⁺ channels in syncytiotrophoblast renewal has yet to be explored. Here we use the placental villous fragment explant model to test the hypothesis that K⁺ channels regulate syncytiotrophoblast renewal. In this model the syncytiotrophoblast is shed over the first 2 days of culture, then regenerates over days 3-6 to form a new syncytiotrophoblast, morphologically similar to fresh placenta, which secretes increasing amounts of hCG (Simán et al 2001). Placentas were collected at term, with informed patient consent. Villous fragments were dissected and maintained in culture for 6 days as previously described (Simán et al 2001). Medium was collected daily for analysis of hCG (ELISA: mIU/h/mg protein) and lactate dehydrogenase (LDH), a marker of cellular integrity, (Cytotoxicity Detection Kit: absorbance units/h/mg protein), and explants treated daily with K⁺ channel modulators, or medium alone, on days 3-6. Data are presented as mean±SE, with n=5 placentas and analysed by 2 way ANOVA with Bonferroni post tests. hCG secretion increased from day 3 of culture, rising from 1.13±0.14 to 3.63±0.69 at day 6, while LDH release was stable at 0.071±0.02 and 0.066±0.01 respectively, coincident with syncytiotrophoblast regeneration previously reported (Simán et al 2001). Tetraethylammonium (TEA: blocker of voltage-gated (K_V) and calcium-activated (K_Ca) K⁺ channels) prevented the increase in hCG secretion over days 4-6, significantly reducing hCG secretion at 5mM (1.61±0.24) and 10mM (1.11±0.28) on day 6 compared with control (p<0.05). 4-aminopyridine (4-AP: blocker of K_V channels) did not alter hCG secretion at 1mM but 5mM significantly inhibited secretion (1.03±0.23 at day 6: p<0.05 vs control). In contrast, the K⁺ channel opener cromakalim (10μM) was without effect (3.41±0.63 at day 6, n=4). None of the K⁺ channel modulators altered LDH release, indicating that the inhibition of hCG secretion by TEA and 4-AP was not caused by loss of tissue integrity. We conclude that K_Ca and/or K_V channels, sensitive to TEA and 4-AP, modulate hCG secretion by placental trophoblast. Modulation may occur either directly, by altering the secretory mechanism, or indirectly, by influencing the cellular turnover required for syncytiotrophoblast renewal.