In the guinea pig the hypothalamus has electrical rhythms independent of that of the cerebral cortex. The rhythms are slow in the anterior and fast in the posterior hypothalamus. The difference may be due to the structural characteristics of the 2 regions. Anesthetics, various drugs and physiological stimuli had little or no effect on the hypothalamic record. Synchronous cortical and hypothalamic barbiturate bursts were not seen. As the result of hypothalamic injury, bursts of grouped high voltage potentials appear simultaneously in the cortex and hypothalamus. Repetitive sine-wave stimulation of the hypothalamus affects the eeg. and produces arousal or activation-type responses and seizure-type responses. Single-shock square-wave stimuli of the fornix, hippocampal commissure or mammillo-thalamic tract constantly produce diffuse responses in the cerebral cortex and cerebellum. Repetitive square-wave stimulation of the same structures induced changes of convulsive character in the cortical and cerebellar records. The possible relation of these changes to those obtained by stimulation of the diffuse thalamic system and the brain stem reticular formation is briefly discussed.