Many systems for rapid perfusion of isolated cells or membrane patches operate with separate capillaries for each drug to be applied. We describe here a simple way by which the solutions in a single capillary can be quickly exchanged during recording. The device consists of a 100 microliters reservoir mounted 20 mm above the preparation and a capillary through which test perfusion media are driven by gravity. The application system is used for preparations attached to a recording pipette and includes two capillaries with solution exchange capabilities within a larger bath tube; it is driven by a capstan motor which is held in a neutral position by a spring. An unlimited number of solutions can be applied through each channel. The volume for washing and refilling a channel is typically 0.5 ml; it may be as low as 0.1 ml. The time interval necessary for sequential applications of different solutions through the same channel is less than 20 s. The solution flows through the capillaries and the bath tube at a speed of about 3 mm/s (30 and 600 nl/s respectively). The mechanical displacement for concentration jumps is adjustable from 0 to 0.4 mm in two directions and can be set to occur within 2-100 ms.