This study is an attempt to detect the most important modifications of physiological parameters occurring during pressure immobility in rabbits and to compare them with those recorded during animal hypnosis. Like the latter, pressure immobility is characterized by the development of high voltage slow waves in the EEG, reduction in frequency and amount of rhythmic slow activity in the hippocampus (RSA) and depression of spinal polysynaptic reflexes. Systolic and diastolic blood pressures are not modified. Duration of two types of immobility is positively correlated within individuals. Treatment by a single dose of morphine (1 mg/kg) potentiates the duration and this effect is antagonized by naloxone (1 mg/kg). Repeated morphine injection up to tolerance reduces duration. Pressure immobility may also be produced under persistent nociceptive stimulation and is characterized by the development of high voltage slow waves in the EEG, as is typical in the absence of pain. Naloxone, (5 mg/kg) injected in a condition of persistent noxious stimulation, reduces immobility duration. In contrast to animal hypnosis, the duration of pressure immobility is neither potentiated by pain nor reduced by naloxone (1,5 or 20 mg/kg). It is suggested that the two immobilities are controlled by several mechanisms, some similar, some different.