Flooding of tomato roots results in decreased stem growth. We have shown that flooding will reduce levels of gibberellins (GA) in the roots, shoots, and bleeding sap of tomato plants. The adventitious roots that appear on the third day of waterlogging may be responsible for the production of GA that accumulate in the shoot after 3 to 4 days of flooding. The endogenous GA of tomato will stimulate stem growth of tomato plants. Initially, application of gibberellic acid (GA3) will stimulate the growth of flooded plants to a greater extent than that of nonwaterlogged plants. It is suggested that one of the first effects of flooding is to reduce GA levels and so inhibit stem elongation. At a later stage of waterlogging GA3 is less effective and other factors appear to inhibit shoot growth.