Normal saline solution is currently used as the ventricular irrigation fluid during neuroendoscopic procedures. The aim of this study is to determine the alterations in the cerebrospinal fluid (CSF) composition after neuroendoscopic interventions. Twenty nine patients who underwent a neuroendoscopic procedure under general anaesthesia were studied. Temperature inside the cerebral ventricle was measured and samples of CSF were taken to determinate oxygen and carbon dioxide partial pressures, pH, base excess, ionised calcium, standard bicarbonate, glucose, sodium, potassium, magnesium, total calcium, proteins, chlorine and osmolality before initiating the irrigation and after the neuronavigation. Patient demographics, neuronavigation time, total fluid volume used and temperature of the irrigation solution and complications that appeared in the first 24 hours were collected. Mean age of the patients was 42+/-18 years. The mean neuronavigation time was 21.5+/-15.4 minutes. The mean amount of saline solution used for irrigation was 919.6+/-994.7 mL. All the values studied in the CSF, except osmolality, showed significant variations. There was a significant correlation between the CSF variation of pH, oxygen and carbon dioxide partial pressures, base excess, standard bicarbonate, glucose and total calcium with respect to the total volume of irrigation solution, but not with respect to the neuronavigation time. A cut-off point of 500 mL of irrigation solution (sensitivity 0.7; specificity 0.87) was related with a CSF pH decrease greater than 0.2. The use of saline as irrigation solution during neuroendoscopic procedures produces important changes in CSF.