The choroid plexuses secrete cerebrospinal fluid (CSF) and regulate the brain's internal environment via the blood-CSF barrier. The permeability properties of the blood-CSF interface have been studied previously in adult and immature brains, however, little is known about the development of CSF secretion and its modulation. ATP influences secretion in other epithelia via ionotropic P2X or metabotropic P2Y receptors. P2 receptors have frequently been found to be down-regulated in the postnatal period, suggesting a developmental role for purinergic and pyrimidine signalling. The present study investigated the expression of P2 receptors in lateral ventricular choroid plexus in relation to recent studies of aquaporin-1 expression and rapid expansion of the lateral ventricles in rat embryos. In the present study mRNAs for all known mammalian nucleotide receptor subtypes, except P2X(7), were identified from as early as E15. P2X(7) mRNA was detected from E18. Indications of differential expression patterns were observed for the different subtypes during development: an apparent increase in expression for P2Y(2) and P2X(7), a decline in P2X(1-2,4), no detectable difference in expression levels for P2X(6) and P2Y(12-13) and transient expression peaks for P2X(3,5) and P2Y(1,4,6,14). P2X(4,5,7) and P2Y(1,4) receptor proteins were detected immunohistochemically in the choroidal epithelium from early in development (E15 or E18). Their differing developmental profiles suggest specific roles in the development of CSF secretion that may have particular relevance for the rapid expansion of the ventricles that occurs in the embryo. P2X(5) and P2Y(6) were also detected in the developing neuropendyma from P0 and P9, respectively.