The new infra-red gas analyser for measurement of CO s concentration described uses a focussed, dual optical path. The 2 W radiation source is a heated alumina bead and a cooled lead selenide photoconductive detector measures the difference in radiation absorption at 4.26 tzm by the gas in sample and reference tubes. Radiation is chopped alternately between these tubes at 120 Hz. The signal from the detector is processed through an a.c. coupled amplifier, phase sensitive detector and low pass filter. Incorporated into the photosynthesis meter, the sample tube of the analyser is connected to a leaf chamber and circulating pump forming a closed gas circuit. As a leaf in the chamber removes carbon dioxide from the air in the closed circuit, the decrease in its concentration is sensed by the analyser. The time taken for the concentration to decrease by a predetermined amount (typically 30 ppm) is displayed and rate of net photosynthesis can be calculated from this and the volume of the dosed circuit. A measurement of the lightsaturated rate of net photosynthesis of a healthy flag leaf of wheat can be made in 10-15 seconds. The system is fully portable and has been used intensively in the field for two summers.