The dynamics of cerebrospinal fluid (CSF) cellular system plays an active role in the homeostasis of the central nervous system, and it obbeys a cibernetic model. In order to verify the occurrence and the frequency of signalisation among the cellular population proper to the CSF, two series of cases were studied. In the first series patients with neurological disorders of several types were grouped (103 cases). The second series groups patients with chronic headache, the neurological examination being normal as well as the CSF examination (30 cases). In both groups the total cell count was normal. Cytomorphology was studied through optical microscopy of preparates obtained by accelerated gravitational sedimentation employing Suta's chamber, and stained by May Grünwald-Giemsa technique. Polinuclear neutrophils, macrophages, plasmocytes and eosinophil cells were found in 71 cases of the first group, among those proper to normal CSF population. The later was the only finding in the remaining 32 cases of the first group and in all cases of the second group. Lymphocytes and reticulomonocytes proper to the normal population were found both in activated and in non-activated stages. No significant differences beteewen the groups were found in this way. So, activation of the CSF lymphocyte-reticulomonocyte cellular system is a finding registered in cases whose CSF space is signalised by conditions proper to the pathologic process as well as in cases with no evidence of such a signalisation.