CSF concentration of cyclic adenosine 3’,5’-monophosphate (cAMP) was measured radioimmunologically from 29 neurological patients before and after injection of air into the ventricular system in pneumoencephalography (PEG). No uniform alteration caused by the procedure in cAMP concentration could be found. In patients with noticeable cerebral atrophy, the cerebrospinal fluid concentration of cAMP tended to be lower than in the subjects with a normal PEG finding.