We describe a temperature-control system for solutions in free flow, suitable for electrophysiological or optical studies of isolated cells, natural epithelia or cell culture monolayers. The system is small enough to be located close to the preparation and was designed specifically to be coupled to the inlets of a modified, continuous-flow Ussing chamber, allowing rapid change of the solutions bathing tissue surfaces. The system consists of a highly compact monoblock heating unit and a control circuit. Solutions from different reservoirs, kept at room temperature or lower (from an ice bath), can be rapidly switched at the inlet of the heating unit by manually or electrically actuated microvalves without affecting the temperature of the fluid leaving the heating unit. The control unit consists of a bead thermistor firmly placed close to the heating unit outlet and an electronic circuit which is basically a proportional controller. This unit continuously regulates the electric current through the Ni-Cr heater, keeping the temperature of the fluid leaving the heating unit constant at a preset value. The system allows control of fluid temperature (normally 37 degrees C) for flow rates in the range of 1.0 ml/min to 12 ml/min. However, the temperature can be set at any value above that of the incoming fluid.