The movement of growing primary roots of maize was measured using a computer driven video-digitizer system. Real time measurements with two perpendicularly mounted video cameras permit measurements of growth movements in three dimensions and allow precise control of growth experiments. The spatial reconstruction and the angular orientation of the oscillating tips revealed that roots fluctuate in growth rate, straight growth and tip orientation. The movements of roots were less uniform than circumnutational activity of shoots and occurred in roots growing in the dark or light, submersed in buffer or suspended in air. The movements led to tip orientation deviating more than 30 degrees from the vertical without causing gravi-reaction. Large angular fluctuations occurred in cycles lasting on average 80 min. and could be observed even in decapped roots. The data suggest that endogenous movements are the result of internal growth regulation, possibly derived from differential sensitivity or adaptation of the elongating tissue. The ability of roots to respond to reorientation seems to depend upon the extent and direction of previous movements. Endogenous movements are likely to contribute to the gravitropic response and to irregular growth under conditions of microgravity.