The mechanisms responsible for the onset of vigorous contractile activity in uterine muscle at the time of parturition, reversing the obligatory quiescence of pregnancy, remain poorly understood. We here describe the expression in myometrium from human and from timed-pregnant rats of an mRNA species encoding a voltage-sensitive sodium channel. The apparent concentration of this mRNA species, as measured by reverse-transcription polymerase chain reaction, underwent an approximately four-fold decline between mid-gestation and term. These results complement earlier reports of increased sodium current expression in mid- and late pregnancy and further support a role for smooth muscle sodium current in late pregnancy and/or labor.