A device for controlling gas concentration as a function of time according to a predetermined program is described. The apparatus is capable of producing any arbitrary time-varying gas concentration at frequencies up to about 4 Hz and can be set to ultra-low frequencies with excellent accuracy and repeatability. The apparatus consists of a single on/off solenoid valve for each gas in the mixture, all controlled by a microcomputer. The valve is rapidly pulsed open and closed, and the concentration of the corresponding gas in the mixture is proportional to the fraction of the cycle for which the valve remains open. Proir to an experiment, numerical values representing the amplitude of the desired concentration wave form at successive time intervals are entered into the computer memory. When the program is run, the microcomputer successively translates each of these values into a pulse-width modulated signal that opens and closes the valves via optically isolated relays. The processor can also translate an external control signal into the corresponding pulse train, permitting manual or automatic control of the mixture. The apparatus can be assembled in several hours from readily available components costing under $600, including the microcomputer.