1. A longitudinal clinico-physiological study in the kinetic tradition has been made on 3 cases of periodic catatonia over a 2-year period. Twice-or thrice-weekly EEG records were analysed mainly for alpha rate, incidence, and amplitude, and for activity in other frequency ranges; daily estimations of the capillary blood-oxygen saturation under both resting and stressful conditions were made by an oximetric technique; and the results of all these measures were assessed in terms of objectively recorded changes in the psychiatric state and of other metabolic happenings similarly investigated.2. In all 3 patients it was found that, starting from a relatively normal baseline in the remission phase, fairly consistent physiological changes occurred with the onset of stupor or catatonic excitement. Both the EEG alpha incidence and alpha amplitude fell sharply and simultaneously with the appearance of a disturbed phase, while the alpha rate rose steeply in 2 patients and decreased in the third at this time. In the periods of semidisturbance, whether these occurred before or after actual stupor or excitement, intermediate changes were seen.3. A similar cycle of disturbance, closely corresponding to the changing psychomotor state, was noted oximetrically. Stupor or excitement was accompanied by capillary anoxaemia and a fall in the stress responsivity score. Both these variables underwent a recovery as the clinical condition remitted.4. The metabolic concomitants of atypical stupor patterns and of the exhibition of thyroid extract were analysed in similar fashion and a tentative hypothesis linking these findings to the patients' varied attempts to achieve an homeostatic equilibrium in the face of stress is offered.Abstract Teaser