Summary: We analyzed the electrical activity of the basolateral amygdala (BLA), anterior and posterior regions of the cingulate gyrus (A-CG and P-CG) , the dorsal hippocampus (DH) , the anterior ventral thalamic nucleus (AVTN), and the sensory motor cortex during the rapid eye movements and ponto-geniculo-occipital (PGO) activity of REM sleep in cats in chronic preparation. Polygraphic recordings and computational peri event averages using the phasic contractions of the lateral rectus muscle (LR) of the eyeball as the triggering signal of the analysis were performed. We observed biphasic potentials (200--300 ms) of variable amplitude, related to the phasic phenomena of REM sleep, in the BLA, A-CG, P-CG, DH, and AVTN. The latencies of the potentials of these regions were always greater than those of the geniculate PGO activities. We propose that the recorded limbic potentials resulted from propagation of PGO activity and that this phenomenon may reflect the limbic structure of the hallucinatory, vegetative, and emotional components of REM sleep. Key Words: Sleep--Amygdala-Hippocampus-Gyrus cinguli-REM sleep--Ponto-geniculo-occipital activity.Jouvet and Michel (1) demonstrated the existence of monophasic spikes in cats at the level of the pons during REM sleep. This electrical activity is propagated from the pontine region (2,3) to the cortical and subcortical structures of the visual system (4--6). Jeannerod et al. (7) were the first to call the phenomenon ponto-geniculo-occipital (PGO) activity. Ponto-geniculo-occipital activity has been studied thoroughly in the oculomotor system (8-12).In man emotional changes, mnestic phenomena of a personal and conceptual type and vegetative changes accompany the rapid eye movements of REM sleep. Jouvet et al. (13), Snyder et al. (14), Baust and Bohenert (15), Welch and Richardson (16), and FernandezGuardiola et al. (17) have described variations in the frequency of both cardiac and respiratory rates related to rapid eye movements.The functions related to emotional and vegetative changes are integrated in the limbic system and in the hypothalamus. Electrical stimulation of these structures elicits emotional