Abstract Guayule ( Parthenium argentatum Gray) is now a commercial crop for the production of high-quality, hypoallergenic natural rubber latex. Because guayule is relatively resistant to both insect and disease pests, its cultivation requires little chemical input. It has been postulated that guayule's chemical defense system is based on terpene derivatives such as guayulins A and B. The goals of this research were to: (1) describe the distribution of guayulins A and B throughout the plant, (2) determine whether guayulins can be used as a predictor of rubber content/yield (is guayulin content related to rubber content), and (3) determine whether the guayulin content/distribution has been changed through plant breeding. Two plants each of three different lines (11591, AZ-1, and AZ-3) and two ages (1 and 2 years old) were harvested at the soil line in the fall of 2002 and spring of 2003. Plants were separated into eight parts: brown leaves, green leaves, stem tips, stems less than 5 mm in diameter, stems between 5 and 10 mm, stems greater than 10 mm, green stems, and flower parts. Samples were analyzed for guayulins A and B and rubber content. Guayulins A and B and rubber were found in all plant parts, but were most prevalent in stems larger than 10 mm in diameter, and were significantly correlated with each other in these stems. Guayulin A was found in greater concentrations than guayulin B. There were no significant differences between plant ages for rubber, or guayulin content at the time of harvest, or guayulins for harvest season. Rubber percent was higher in spring, which agrees with numerous other investigations. Guayulin A in the stems is correlated with the total rubber in the plant, suggesting it as a potential selection tool. However, rubber in the same plant parts has a higher correlation with total plant rubber and remains a better and easier selection criterion than guayulin A. The extent to which guayulin contents differed between older, relatively unimproved lines and newer improved lines was evaluated. Variety 11591 had a higher concentration of guayulin A than guayulin B, so that the ratio of A to B was much higher than in the two improved lines (AZ-1 and AZ-3). It appears that the ratio of guayulin A to guayulin B has been changed in the two newer selections, but it is unclear whether this will affect pest resistance.