1. Guayule nursery-transplants were grown for 14 months in alternating periods of low and high moisture stress of different durations. The dry weights and the carbohydrate, resin, and rubber contents of the plants were determined at the end of each period of moisture stress. During periods of high moisture stress, the rate of increase in dry weight of the entire plant was depressed, but most of this could be accounted for by the effect on leaf weight. The percentages of levulins, in roots and stems especially, during these periods increased markedly, while the percentages of the other carbohydrates-sugars, inulin, and pentosans-remained fairly constant. Except for the period from November 6 to March 6, the percentages of rubber increased during periods of high stress at a more rapid rate than during periods of low stress. 2. During periods of low stress following high stress, the percentage and absolute content of levulins per plant decreased. This indicated that the levulins were serving as the principal...