The rate of auxin transport in sunflower hypocotyls (Helianthus annuus L. cv ;Russian mammoth') or corn coleoptiles (Zea mays L. cv ;WF9 x 38') was less in seedlings grown in Ca-deficient medium than in controls. The rate of IAA transport depended on the concentration of Ca in the root medium up to 1 millimolar. Further increases in auxin transport were observed when the isolated segments were incubated in medium containing up to 30 millimolar Ca. We suggest that the rate of auxin transport in plant tissue is dependent on the pool of ionic Ca in the extracellular space.Segments from Ca-deficient seedlings exhibited a high specific requirement for Ca(2+) in auxin transport. Magnesium, strontium, and several other divalent cations tested for their ability to replace Ca(2+) in restoring auxin transport showed no effect; partial replacement by lanthanum was observed.Auxin transport, or auxin flux through the segment, which is the result of IAA secretion by individual cells, was reduced in the low Ca(2+) segments due both to lowered velocity and to reduced capacity of transport. The requirement for Ca(2+) in the secretion of auxin is believed to be equivalent to the phenomenon observed in animal cell secretion, where the influx of Ca(2+) serves as a link between an external stimulus and the secretion response.