A system is described that allows to perfuse in vitro isolated mammalian brain preparations obtained from adult guinea-pigs. It consists of a microprocessor controlled thermoregulation unit, a perfusion line and a perfusion/recording chamber. The main features of the thermoregulation unit include the ability to maintain stable temperatures between 10 and 37 degrees C and to precisely control the time course of the temperature changes. Using this system, perfused tissue blocks can be kept viable for 8-10 h.