The influence of several natural and synthetic hormones on apical dominance of pea roots was investigated. Extracts of roots decapitated just before extraction or 12 h earlier were compared in a lateral root formation assay, in the Amaranthus assay for cytokinins and the Avena coleoptile assay. Indole-3-acetic acid shows a strong promotive effect on the initiation of lateral roots. Gibberellic acid has no significant effect. Abscisic acid and its presumable precursor xanthoxin show a strong inhibitory effect, and so do all cytokinins investigated (kinetin, benzyladenine, and zeatin). Decapitation of roots caused a significant increase in the number of lateral root primordia within 12 h. Paper chromatograms of root extracts indicate the presence of at least two inhibitors of lateral root formation in the alkaline fraction, and of two promoters in the acid fraction. The amount of both inhibitors seems to decrease if the tip of the main root is removed 12 h before extraction whereas the promoters of lateral root formation do not change in their concentration. The Amaranthus test shows cytokinin activity in the region of one of the two inhibitors, and this activity decreases after decapitation. The second inhibitor has the Rf of xanthoxin; it inhibits the growth of Avena coleoptiles and its activity also decreases after decapitation.