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 P2X7, were identified from as early as E15. P2X7 mRNA was detected from E18. Indications of differential expression patterns were observed for the different subtypes during development: an apparent increase in expression for P2Y2 and P2X7, a decline in P2X1-2,4, no detectable difference in expression levels for P2X6 and P2Y12-13 and transient expression peaks for P2X3,5 and P2Y1,4,6,14. P2X4,5,7 and P2Y1,4 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. P2X5 and P2Y6 were also detected in the developing neuropendyma from P0 and P9, respectively.