The mammalian choroid plexus consists of meningeal tissue containing numerous thick-walled capillaries covered by a single layer of cuboidal epithelium 12. Noradrenergic and possibly chotinergic nerve fibers and terminals have been identified by histochemistry within its parenchyma and around blood vessels where they may modify the rate of formation of cerebrospinal fluid (CSF) a, 9. Stimulation or destruction of the superior cervical ganglia reportedly decreases or increases, respectively, the rate of CSF production, and modifies the activity of the plexus enzyme carbonic anhydrase 2. Other myelinated and some unmyelinated neurons are also located within the choroid plexus 15 and, although their origins and functions have not yet been determined, it has been suggested that they also modulate the secretory functions of this tissue. In this report, we provide biochemical evidence that:(1) the choroid plexus contains measurable amounts of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT);(2) this molecule is probably localized within nerve terminals that arise from neurons within the dorsal and/or medial raphe nuclei, and (3) levels of this neurotransmitter can be raised or lowered by the administration of drugs that modify either its synthesis or degradation, or that destroy serotonin-containing neurons by a mechanism dependent upon its active uptake across membranes. Male Sprague-Dawley rats were maintained at an ambient temperature of 22 C under diurnal lighting conditions (08.00 to 21.00 h) and fed Purina chow ad libitum. In each of the experiments outlined below, animals were perfused with cold saline (4 C) to remove blood elements from the choroid plexus prior to decapitation. To accomplish this, ether-anesthetized animals were subjected to right auriculectomy and then perfused via the left ventricle with 75 ml of cold saline. Animals were decapitated and their brains were placed on chilled glass plates. The choroid plexus was removed from the lateral and third cerebral ventricles via an incision in the overlying cerebral cortex. Tissue was frozen on dry ice and stored at--20 C until assay. Serotonin was measured by a specific and sensitive microassay developed in our laboratory, in which serotonin was first N-acylated with hepatic, rodent N-acetyltransferase (EC 2.3. 1.5) and subsequently 5-methylated with bovine, pineal hydroxyindole-O-methyltransferase (EC 2.1. 1.4) using