Apical segments of roots of Zea mays L. cultivars Orla and Anjou show a strong georeaction during 7 h geostimulation. This is abolished by detipping the segments and restored by replacing the tips upon the apical cut surfaces. After exodiffusion of endogenous IAA the retipped segments showed a significantly lower geocurvature. Application of low concentrations of IAA to the basal cut surface of root segments, from which endogenous IAA had not been allowed to exodiffuse, increased the geocurvature of retipped Orla segments but decreased geocurvature of ''Anjou'' segments. At appropriate concentration basally applied IAA restored the georeaction capacity of root segments from which the endogenous auxin had exodiffused. The implications of the interaction between exogenous and endogenous IAA in the control of root georeaction are discussed; special reference is made to the normal role of endogenous IAA in the regulation of root georeaction and the variation in endogenous IAA content of roots of different cultivars of maize. The probability is stressed that the normal control of root growth and georeaction involves concomitant actions in the elongation zone of IAA moving preferentially in the acropetal direction and of basipetally transported growth inhibitors (such as abscisic acid) produced in the cap cells.