The effect of the 1-24, 1-17, 1-13, 4-11, 1-4 ACTH fragments on cortical and hippocampal electrical activity, behavior, heart rate and rectal temperature of non-anesthetized rabbits was studied. All the fragments proved to be active in modifying cerebral electrical activity of both structures, although no common point of attack based on our data could be hypothesized. Animal behavior was not significantly modified. On the contrary, changes in rectal temperature were observed in all the animals treated. The hypothermal effect seemed to be related to the 11-13 aminoacid sequence.