Greater than 30% mortality was observed unexpectedly in rat pups of weaning age kept with their dams in individually ventilated cages (IVC). A subsequent controlled study involving an induced electrical power failure that halted ventilation of IVC containing rat dams and pups showed that oxygen concentrations in the air in individual cages were rapidly depleted to less than 10% in 1 h. In a separate incident during which electrical power to IVC was inadvertently disconnected, rats died within 60 min of loss of power to IVC. The loss of electrical power to IVC housing rats is potentially a life-threatening condition requiring an immediate response and affirms the consideration of emergency (back-up) electrical power where these systems are used.