We previously demonstrated that fast Na+ channels are expressed in uterine smooth muscle cells during pregnancy [Y. Ohya, and N. Sperelakis. Am. J. Physiol. 257 (Cell Physiol. 26): C408-C412, 1989; Y. Inoue, and N. Sperelakis. Am. J. Physiol. 260 (Cell Physiol. 29): C658-C663, 1991]. In the present study, we investigated the possible expression of fast Na+ channels and resultant fast Na+ current [INa(f)] using whole cell voltage clamp in cultured uterine leiomyosarcoma cells (SK-UT-1B), which are derived from human myometrium. The expression of INa(f) was dependent on the concentration of fetal bovine serum (FBS) in the culture medium. The mean current densities of peak INa(f) in cells incubated with 0, 1, 5, and 10% FBS were (in pA/pF) 1.3 +/- 0.5, 1.6 +/- 0.9, 9.8 +/- 2.4, and 9.8 +/- 1.7, respectively. Two types of INa(f) were identified based on different time courses of current decay. Both types of current were equally tetrodotoxin sensitive (half-maximal inhibitory concentration approximately 50 nM) and had similar electrical properties. These characteristics are consistent with those in the pregnant myometrial cells, yet the current density in the sarcoma cells is much higher. These findings suggest that one or more factors in serum induce INa(f) in the uterine sarcoma cells, and this cell line could be a good model for the study of the fast Na+ channels in uterine smooth muscle.