• The role of dibutyryl cyclic adenosine 3',5'-monophosphate (db cAMP) in influencing the water content of the cat's gray and white matter was evaluated by intracortical injection of 20 μL of 2 x 10^-1 to 2 x 10^-6M solution of db cAMP. Cholera toxin, a stimulator of adenylate cyclase, also was tested. Concentrations of db cAMP less than 10^-3M failed to produce significant change in brain water content, while concentrations greater than 10^-2M produced 31.5% ± 8% and 17.3% ± 3.6% increases in white matter volume, respectively (P <.05). Cholera toxin did not increase brain water levels. These results are discussed in relation to the pathophysiology of brain edema.