The effect of pH on the elongation rate of 2-mm-long subapical segments from corn roots (Zea mays L.) was investigated under aerobic (O2) and anaerobic (N2) conditions using a specially designed apparatus. The maximum elongation rate was obtained with a citrate-phosphate buffered solution of pH 3.4 or a non-buffered solution of pH 3.2. Acid stimulation of the elongation rate occurred in the zone of maximum elongation in the root, the 3 mm region 1 mm behind the apex. The acid response can be rapidly and repeatedly reinitiated by alternating between pH 4.0 and 7.0 media. Low pH can increase the elongation rate under N2 conditions, but the response is only ca. 50% that of the increase observed under O2 conditions. Reinitiation of the acid response does not occur when solutions are gassed with N2. The data suggest that the response of root segments to H ions is similar to that reported in the literature for shoot tissue. However, the magnitude of the root response is much higher than that of shoot tissue. Also, there is evidence that some aspects of the mode of action of H ions on wall loosening in root tissue may differ from that in shoot tissue. Corn root segments, though lacking a cuticle, do not respond to low pH in the manner reported for peeled stem tissue, the cuticle and epidermis of which have been removed. For these reasons rapid-growth responses of roots and shoots may not be strictly comparable.