The effect of ethylene on abscission of flower pedicels of tobacco plants has been investigated. For the first 2 h of exposure to C2H4, the pedicels bend rather than break in response to applied force, but after 2.5 h exposure they break at the abscission zone under an applied force of 40 g. The break strength of the abscission zone decreases exponentially with time to 5 g at 5 h after beginning of the C2H4 treatment. An examination of the tissue at the fine structural level 2 h after exposure to C2H4 reveals the accumulation of rough endoplasmic reticulum (RER) in the abscission cells. Rough ER becomes increasingly abundant by 3-5 h exposure of the tissue to C2H4. There is approximately a 30 fold increase in RER by 5 h of exposure, as compared to untreated tissue.Loss in the integrity of the membranes of microbodies occurs by 5 h exposure of the tissue to C2H4. As cell wall degradation proceeds, fibrous material, vesicular structures, and electron dense bodies-the latter often appearing striated-develop in the disintegrating wall. Little change is seen in the structure of nuclei, mitochondria, chloroplasts and in the crystalloid cores of microbodies during the first 5 h of exposure of the tissue to C2H4. However, disorganization of cytoplasmic components does occur in cells where cell wall breakdown is at an advanced stage.