Autoradiographic studies showed that regardless of whether 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone (pyrazon) was applied to the leaves or to the roots of sugar beet (Beta vulgaris L.) plants, it moved in the apoplastic system. The pattern of pyrazon distribution from root absorption in sugar beet seedlings was identical at either 35 or 18.3 C. However, root absorption at 35 C was twice as great as at 18.3 C; and translocation of pyrazon into the shoot was more rapid at the high temperature. A major metabolite of pyrazon, a pyrazon-glucose conjugate, was produced in leaves and cotyledons but not in roots of sugar beets. A minor metabolite, less than 5%, was found in sugar beet leaves. Pyrazon was not metabolized by the susceptible species common lambsquarters (Chenopodium album L.). The rate of pyrazon-glucose conjugate formation in pyrazon-infiltrated sugar beet leaf discs was practically identical at 35 and 18.3 C. Therefore, it was concluded that the increased susceptibility of sugar beets to pyrazon at higher temperatures was due to an increase in absorption and translocation of the herbicide at higher temperatures which was not accompanied by a parallel increase in the conversion of pyrazon to its glucose conjugate.