Guayule (Parthenium argentatum) is currently under development in the southwestern United States as a source of natural rubber, resins, and biomass to supplement the tropical supply of natural rubber from rubber trees (Hevea brasiliensis). Knowledge of the fundamental biology and biochemistry of natural rubber synthesis in plants will lead to practical approaches to improve yield. In this investigation, living rubber particles were purified from guayule and Hevea capable of synthesizing rubber in in vitro enzymology experiments. Concentrations of initiator molecules and monomer molecules were varied, and rubber production measured over time. Under most conditions, it took between 1.5 h and 4 h to produce mature rubber polymers; but surprisingly, guayule could synthesize long polymer chains in under 15 minutes. Molecular weights, as calculated from substrate incorporation, varied from 10,000 to over 10 million g/mol, even though the product found in nature is usually about 1 million grams/mol. The data also reveal some insight into chain termination - a poorly understood phenomena in vivo. Finally, results inform models of the structure of the rubber transferase complex that may be useful for plant engineering or synthetic biological approaches to rubber production.