The concentrations of Cl, Na, K and inorganic P were determined in CSF samples of 50 patients with epilepsy. In 33 patients the sample studied was collected at the occasion of a convulsive seizure (group I). In 17 patients it was collected in the intermediate period between seizures (group II). The patients of group I were children. They were grouped according to conditions associated to the convulsive seizure. In 9, there were no other manifestation of disease (sub-group Ia). In 12, the seizure was associated to hyperthermy (sub-group lb) and in 12 it occurred during rehydration therapy (sub-group Ic). The results were compared statistically to those obtained for a series of 25 non epileptic patients taken as control (group III). The results obtained for the latter fall in the normal range and were reported previously. Results obtained for the patients of the group II did not differ from those found in the control group. The concentrations of K and inorganic P were found to be increased in group I. Their mean differ significantly of those found for control group. There occurred a positive correlation between the concentrations of K and P in group I. The value of the correlation was significant. The findings regarding to the concentration of Na show shiftings that were not uniform. Their mean value was elevated in the sub-group Ia, normal in the sub-group Ib and low in the sub-group Ic. The mean value of the Cl concentration was low in the sub-group Ic. It was normal in the other two sub-groups. Data concerning to the changes in the concentrations of K and inorganic P in the CSF during convulsive seizures were discussed. Bioelectric changes of the cellular membrane during seizures are reported in the literature. They induce the passage of these components through the cellular membrane to the extracellular space thus resulting the elevation of their concentration in this space. The results presented make it possible to evaluate the extension of such changes in the fluid of the extravascular space. The latter is represented in the case by the CSF. The exchange of intracellular K and inorganic P to the extracellular space is accompanied by exchange of Na in the opposite direction. The Na concentration is referred to elevated in the intracellular space in these circumstances. The results reported suggest that Na is not provided by CSF only, for this exchange. Other compartments of the extracellular space and/or of the intracellular space may be more directly involved. This is suggested because the concentrations of Na in the CSF did not show uniform changes in the three sub-groups of patients with seizures that were considered. General changes in the extracellular fluid composition might explain the differences in the concentration of Na which were reported. The concentrations of Na and Cl found in the CSF of patients with convulsion during rehydration therapy are elucidative in this way. They were found to be low, suggesting an excess in the water uptake.