The role of extracellular Ca(2+) in root-hair tip growth has been investigated in Arabidopsis thaliana (L.) Heynh. Root-hair length was found to be dependent on the concentration of Ca(2+) in the growth medium, with maximum length achieved at [Ca(2+)] of 0.3-3.0 mM. Using a non-intrusive calcium-specific vibrating microelectrode, an extracellular Ca(2+) gradient was detected at the tips of individual growing root-hair cells. The direction of the gradient indicated a net influx of Ca(2+) into root-hair cells. No gradient was detected near the sides of the root hairs or at the tips of non-growing root hairs. When root hairs were exposed to the Ca(2+)-channel blocker nifedipine, tip growth stopped and the extracellular Ca(2+) gradient was abolished. These results indicate that Ca(2+) influx through plasma-membrane Ca(2+) channels is required for normal root-hair tip growth.