In 4 patients slight emotional stimulus (laughter, surprise) produced occasional unpredictable fits of involuntary immobility, without loss of consciousness. The accompanying symptoms (loss of breath, cardiac palpitations, subjective sensation of heat or cold, pallor, perspiration, etc.) suggest a primarily thalamic localization, with immediate spread to cortex in part and to hypothalamus in part. The author suggests that the phenomenon may be a biological defense reaction (akin to the immobility of the frightened animal) which has lost its biological significance by becoming attached to indifferent emotions.