Light-and electron-microscopic studies of the distribution of calcium in gravitropically responding oat (Avena sativa L. cv. "Garry") coleoptiles are described. A modification of the antimonate precipitation procedure was used to localize tissue calcium in situ. An accumulation of Ca in the upper halves of horizontal, gravistimulated coleoptiles is seen within 10 min of stimulus onset. A pronounced redistribution of Ca to the upper side occurs within 30 min; although the localization of this cation is not uniform along the organ axis and in the apical region, Ca appears to accumulate along the lower side. The observed asymmetric distribution of Ca in these tissues precedes large-scale visible bending by 20-30 min, but is temporally well-correlated with differential growth responses in the coleoptile, as measured by more sensitive quantitative techniques. Gravitropic curvature is well developed by 3 h and is accompanied by further redistribution of Ca to tissues along the upper coleoptile half, centered around the bend. Ultrastructural localization studies indicate that Ca asymmetry results primarily from changes in the distribution of Ca within the apoplastic compartment. Large amounts of Ca accumulate at the cuticle in epidermal cell walls and in the walls of the underlying parenchyma cells at the upper side of the organ in the region of maximal bending. The differential growth response resulting in the establishment of gravitropic curvature may largely be the consequence of antagonistic effects of Ca on auxin-mediated cell wall loosening and elongation growth processes at the upper side of the organ.