Remotely-sensed observations of visible and near-infrared canopy reflectance were used to monitor the response of cotton to elevated CO₂ concentrations during the 1989 FACE Experiment. Data were collected several times each week using a portable, handheld radiometer along permanent transacts in four enriched arrays (550ppm CO₂) and paired controls (ambient CO₂). A spectral vegetation index (VI) was computed as the ratio of NIR and Red reflectances. The VI was sensitive to the amount and condition of green plant material viewed by the radiometer. Differences in growth, leaf area and biomass between enriched and control cotton plants were evident in the temporal VI values. The non-destructive spectral measurements were useful for quantifying plant response to experimental treatment variables between biweekly, destructive samples of plant material. These techniques can be used in a near-real time fashion to supervise conduct of the experiment.