Summary • Because of its small size and rosette growth habit, measuring gas exchanges in Arabidopsis thaliana is difficult with standard leaf cuvettes. Here, we designed a versatile system that is usable at the whole rosette level, as small as possible for fast and accurate measurements, but adaptable to plant size, and suitable for in situ measurements whatever the growing substrate of the plant. • This cuvette is in two parts: the basic unit, which contains the sensors and is connected to the infra-red gas analyzer, and the clear chamber, where the rosette is enclosed. • We made a set of three interchangeable chambers of different sizes to measure the rate of CO2 assimilation [A] of 26-, 33- and 40-d-old plants. The dependence of A to light irradiance and to intercellular CO2 concentration was recorded as typical response curves, which validate our device. • Measurements were not only consistent in saturating conditions, but accurate CO2 exchange measurements in limiting conditions also reflected important physiological features related to plant ageing. ©New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01000.x