We have developed a technique for making a rapid solution change, whilst at the same time maintaining the temperature of the preparation at 37 degrees C. It is technically difficult to use rapid solution changes when experiments are performed at normal mammalian body temperature. As a solution is heated from room temperature to 37 degrees C, gas bubbles form in the rapid-flowing solution streams, and these disturb a cell or attached recording pipettes. We describe a system that has been developed to eliminate these problems. We show how to construct the different components of the system, and we have designed an electronic circuit to control solution changes. We have performed tests to characterise the function of this system. Solution flow out of the nozzle of the device (0.88 ml min-1, linear flow velocity 11.6 cm s-1) caused a fall in the steady-state temperature at the experimental preparation of only 0.3 degrees C. The device which takes between 0.5 and 1 s to completely change the superfusate of a single cell, was used routinely with five different experimental solutions. This system may be valuable in studies which require rapid solution changes to be performed at a normal mammalian body temperature.