The objective was to identify physiological limitations to the consistency in performance of Accelerate^TM (endothall; Elf Atochem) as a cotton (Gossypium hirsutum L.) defoliant. Arrays of droplets of preparations containing ¹⁴C- or technical-grade Accelerate were applied to leaves on mature plants. The removal of ¹⁴C-Accelerate from the leaf surface during 24 h (used here as an estimate of uptake) varied by leaf position in the canopy. Ammonium sulfate enhanced uptake. Accelerate efficacy is greater with a late-afternoon application than with earlier ones. A synergism in activity exists between Accelerate and Scythe (Mycogen).