Intrinsic and dynamic aspects of the sleep-wakefulness cycle (SWC) are studied. From a dynamic point of view, results indicate a tendency to maintain vigilance (alerta wakefulness-W-and drowsiness-D) or sleep (slow sleep-SS-and paradoxical sleep-PS). Transitions from vigilance to sleep are more probable than the reverse. Drowsiness plays an important role as a distributor stage in the SWC of the normal cat since the same probabilities of passing from it to W than to SS exist. Furthermore transitions from sleep (SS and PS) were more frequently observed to D than to W.