The determination of natural rubber and resin contents of guayule (Parthenium argentatum) requires a long and destructive methodology to be implemented in the laboratory. In order to achieve a large number of measurements, it is necessary to determine the contents directly in the field in a fast and non-destructive way. The recent emergence of portable near infrared spectroscopy makes possible the use of this technology directly on the plants, without preparation of samples in the laboratory. The aim is to check the possibility of quantifying natural rubber and resin by using such a portable device directly on the guayule plants. Our research hypothesis is that the measure in near infrared can be performed directly on the surface of the bark. A set of 200 branches of guayule was collected randomly in a single experimental plot for two selected genotypes showing very different morphological traits. A difference between average contents of natural rubber and resin measured using a solvent-based laboratory reference method was observed with relation to the variety. Models of near infrared spectroscopy have been developed for samples in the form of powders, dry branches and fresh branches harvested in the same plot. The standard error of prediction was two times higher for branches as for powders. This was due to the fact that the powders were analyzed in controlled conditions in the laboratory (moisture, sample homogeneity and particle size). Using near infrared spectroscopy directly on the plant allowed the determination of the average content of the plot before harvest. With a random sampling of 70 measures per plot, it was possible to estimate the average content of natural rubber and resin with a precision lower than 0.3%. Measurements made on fresh branches of very different diameters showed that the variability of the contents was lower in the plant than in the field. At the individual plant level, 40 measures were necessary to determine the average content of re