Abstract 1. The concentration of endogenous cAMP was found higher in the nuclear fraction than in the mitochondrial fraction in all regions of rat brain. 2. Incubation with theophylline and F − ions resulted in a distinct increase of cAMP concentration in fractions N and M, mainly in cerebral cortex and hippocampus. 3. The homogenate and both subcellular fractions from thalamus with hypothalamus showed no increase of accumulation of cAMP but sometimes a decrease of cAMP level when treated with theophylline and F − ions.