Foliar-spray and radioisotope herbicidal studies were conducted near Tucson, Arizona, to study factors influencing absorption, translocation and toxicity of (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) in creosotebush [Larrea tridentata (DC.)Cov.]. Maximum absorption and translocation of 14C-labeled 2,4,5-T occurred approximately 30 days after initiation of effective summer rains and coincided with maximum total killing of creosotebush. This occurred during the full-flowering to mid-fruiting stage of phenological development, overlapped the period of old leaf drop, and was due in part to the generally greater basipetal translocation of radioactive 2,4,5-T from old leaves, flowers, fruits, and young bark than from young leaves. Translocation of radioactive 2,4,5-T was relatively slow, 1 to 5 cm per hour, and basipetal movement ceased after 18 to 24 hr, following which the 2,4,5-T largely disappeared from the vascular tissues of the stem. Absorption of 2,4,5-T by the leaves was low during dormancy and may have limited herbicidal effects during this period, but it was not a factor during the season of high susceptibility.