Mast cells are found in the central nervous system (CNS) as well as in the periphery. In the brain of mice, they are localized primarily in the thalamus and meninges. Although their numbers increase in response to stress, the mediator of their recruitment is not known. During studies in which drugs were delivered intrathecally in a volume sufficiently large to distribute to the brain, we discovered that repeated daily injections of this large volume increased the number of mast cells in the thalamus. The increase was not due to changes in electrolyte composition of the cerebrospinal fluid (CSF) as chronically administered artificial CSF produced similar effects. Repeated injections of even small volumes (2 mul) increased mast cells in the medial intralaminar (Med), ventral posterior (VP) and posterior (Po) nuclei. Increasing the volume injected daily to 20 mul increased mast cells in the lateral intralaminar (Lat), laterodorsal (LD), ventrolateral (VL) and lateral geniculate (LG) nuclei and further increased those in the lateral extension of the Po nucleus. Thus, small and large volumes augment distinct populations of mast cells. While stem cell factor (SCF) is abundant in the CNS and is chemotactic to mast cells in the periphery, thalamic mast cells in the rodent do not express c-kit, the SCF receptor, suggesting that this factor may not be responsible for the effect. Consistent with this, centrally injected SCF was incapable of increasing thalamic mast cell populations after either single or chronic (21 days) daily injections compared to the effect of saline alone. Although the mechanism is not known, repeated injections of a large volume of fluid dramatically increase mast cells in the CNS, a phenomenon that may be relevant to clinical conditions of increased CSF pressure or volume.