The progress of bean petiole abscission has been followed using quantitative measurements of the mechanical force required to break explants at the separation zone. It is found that the shortest time for a measurable effect of ethylene (1 ppm) in stimulating the development of frangibility is about 1 hr. Removal of the ethylene is followed by a return to the endogenous rate of weakening, the slower rate being established in 1 hr. Application of the inhibitor cycloheximide leads to a cessation of abscission development within one-half hr. As in the stimulations of certain plant processes with auxins, gibberellins, and cytokinins. ethylene stimulation of abscission requires the continuous presence of the regulator.