Unihemispheric and bihemispheric sleep deprivation were performed in bottlenose dolphins. One brain hemisphere was capable of being deprived of delta (0.5-3.0 Hz) sleep in the former condition. Here, an increase in sleep pressure was observed during sleep deprivation in the deprived hemisphere. In the recovery sleep, following unihemispheric sleep deprivation, there was a rebound of delta sleep only in the deprived hemisphere. Following bihemispheric sleep deprivation the animals exhibited an increase in delta sleep in both hemispheres.