The effect of carbamazepine treatment on CSF-somatostatin-like immunoreactivity (SLI) in patients suffering from temporal lobe epilepsy was investigated. A baseline lumbar puncture was performed on 12 patients and 10 normal volunteers. A second tap was repeated only in patients when they were on peak of carbamazepine concentration for 10 days. Levels of CSF-SLI were measured by RIA. No significant differences were found in CSF-SLI basal concentrations between epileptics and controls, whereas a significant decrease (p less than .0002 Duncan's multiple range test) of CSF peptide levels occurred in 9 of 12 patients under medication. Although the neural mechanism through which carbamazepine lowers CSF-SLI is still unknown, the results of the present study suggest that the reported effect might be part of the apparatus by which carbamazepine exerts its anticonvulsant action.