Non-lethal dosages of 2,4-dichlorophenoxyacetic acid (2,4-D), which stunted pigweed plants, markedly reduced transpiration. Lethal dosages of 2,4-D, 2,2-dichloropropionic acid (dalapon), or toxic oil reduced transpiration rapidly, but transpiration continued until treated plants were air dry. Large plants died slower than small plants after herbicide treatment; consequently, they transpired more water before death.