The influence was studied in vitro of certain agents (adenosine, ADP, ATP, theophylline, together with F- ions) on the cAMP concentrations in the nuclear (N) and mitochondrial (M) fractions from different areas of rat brain. F- ions caused a slight decrease of the cAMP concentrations in nuclear fractions of the thalamus with hypothalamus and a marked decrease of this cyclic nucleotide in M fractions from the cerebral cortex. After incubation with adenosine and F- ions a distinct decrease of cAMP level was observed in N fractions from the midbrain and thalamus with hypothalamus and in mitochondrial fractions obtained from all the investigated regions. The incubation with ATP and F- ions resulted in a distinct decrease of cAMP values in the nuclear fractions from all regions. The concentrations of cAMP in the mitochondria of the midbrain and thalamus with hypothalamus incubated with ATP and F- ions increased 2-3 times. The incubation of the nuclear fraction with theophylline and F- ions caused an increase of cAMP concentration in the cortex and a decrease of cAMP values in the midbrain. The level of cAMP after the incubation with theophylline and fluoride on the mitochondrial fraction is increased in the cortex and decreased in the thalamus with hypothalamus.