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The effects of drought stress and season on both allocation of photosynthates to stems and leaves and potential for stem rubber synthesis were studied in guayule (Parthenium argentatum Gray USDA line 11604). Two-year-old plants grown under field conditions in the Negev desert of Israel were subjected to different irrigation regimes, and water status was assessed by measuring the relative water content RWC). Undetached plant tips were exposed to a 1 h pulse of (CO2)-C-14 followed by a 24 h chase. C-14 fixed and translocated to different plants part and notably C-14 incorporation into rubber and resin fractions was determined. The potential of detached branch slices to incorporate [C-14]-acetate into rubber was also studied. A higher percentage of fixed C-14 was translocated from shoot tips in winter (28-30%) than in summer (15-18%). The percentage of [C-14]-acetate incorporated into the rubber fraction by stem slices was maximal in winter (20%) and minimal in summer (3-5%) in both cases in the absence of drought stress. In summer the translocation of photosynthates into stems was inversely related to plant RWC, dropping from 18% three days after irrigation to 3% 14 days later, and the potential of stems to synthesise rubber was high under drought conditions and low in well irrigated plants.