The effects of a guided neutral and unpleasant imagery involving several sensory modalities were studied in hypnotized subjects. Heart rate (HR), respiratory frequency (RF), tonic skin resistance and different electroencephalographic rhythms were evaluated during a long-lasting hypnotic session including the guided suggestion of a neutral (NS) and an unpleasant (US) imagery, each preceded by a hypnotic relaxation rest period. During NS, the absence of autonomic changes, associated with electroencephalographic gamma power decrement and theta1 power increment, indicated the prevalence of relaxation on the expected task-related modifications. In contrast, US elicited HR and RF increments together with higher electroencephalographic gamma, beta3 and beta2 activities. Thus, hypnotic state appears to prevent the autonomic responses expected during the neutral stimulation, while the emotional valence of the unpleasant imagery overwhelms the hypnosis-related relaxation.