Sphingosine-1-phosphate (Sph-1-P), a product of sphingomyelin metabolism, can act via a family of cognate G-protein-coupled receptors, or as an intracellular second messenger for agonists acting through their membrane receptors. In view of the general growth promoting and developmental effects of Sph-1-P on target cells, we hypothesized that it plays a role in adaptation of the uterus to pregnancy. We analyzed its potential role, and that of the related lysophospholipid, lysophosphatidic acid, in the pregnant rat uterus by examining changes in mRNA levels of cognate receptors and enzymes involved in their turnover. Of these, only sphingosine kinase 1 (SphK1) was markedly changed (about a 30-fold increase), being localized in the glandular epithelium, vasculature and the myometrium. Uterine SphK1 mRNA and protein levels paralleled those of serum progesterone, and treatment with progesterone or an antagonist elevated or reduced SphK1 mRNA expression, respectively. Progesterone also increased SphK1 mRNA steady state levels in a rat myometrial/leiomyoma cell line (ELT3). Overexpressing human SphK1 in these cells resulted in increased levels of the cell cycle regulator, cyclin D1, and increased myosin light chain phosphorylation. Ectopic expression of SphK1 also resulted in increased proliferation rates, possibly in conjunction with increased cyclin D1 expression. These studies suggest that the uterine expression of SphK1 mediates processes involved in growth and differentiation of uterine tissues during pregnancy.