The influence of the spectral distribution of illumination on the gibberellin, cytokinin, auxin, and abscisic acid levels and the correlation with the degree of branching in Xanthium strumarium is presented and discussed. Gibberellins do not appear to play a major role in apical dominance but may be important for bud extension following the initial release from dominance. The cytokinin level was much higher in inhibited buds than in released buds. It is suggested that the cytokinins present were probably not able to participate in bud growth because of an auxin-induced accumulation of abscisic acid in the buds themselves. The concentration of abscisic acid as measured by bioassay and gas-liquid chromatography was between 50 and 250 times that occurring in all other plants parts examined. This level fell dramatically following release from apical dominance by decapitation. The results are discussed in relation to current hypotheses of apical dominance.