Postmortem examinations were made on 240 of the 459 cases succumbing 52 percent of the deaths in the Collaborative Study on Cerebral Survival; the central nervous system was examined in 226 cases. The autopsy was performed on an average of 15.3 hours after death. The mean weight of the brains was 1450 ± 196 grams; the mean weight of the brains of patients on whom resuscitation was stopped, presumably on the basis of “cerebral death,” was greater than that of the patients succumbing to cardiac failure. There was a tendency for the brain to increase in weight about 24 hours after the initiation of resuscitative measures. At that time, swelling, discoloration, softening, congestion, and brain herniations also became more prominent. On the basis of a survey of American neuropathologists and the data from this study, the entity commonly termed “respirator brain” may be confirmed. This is a dynamic process that is complicated by concurrent postmortem changes. The respirator brain requires time approximately 24 hours for maturation; many patients die a cardiac death during the metamorphosis. If the patient survives for 3 to 4 days, the percentage dying with typical respirator brains is less, and more patients have electroencephalograms with biological activity.