When Chlamydomonas cells are deflagellated by pH shock or mechanical shear the nucleus rapidly moves toward the flagellar basal apparatus at the anterior end of the cell. During flagellar regeneration the nucleus returns to a more central position within the cell. The nucleus is connected to the flagellar apparatus by a system of fibers, the flagellar roots (rhizoplasts), which undergo a dramatic contraction that coincides with anterior nuclear movement. A corresponding extension of the root system, back to its preshock configuration is observed as the nucleus retracts to a central position. Anterior displacement of the nucleus and flagellar root contraction require free calcium in the medium. Nuclear movement and flagellar root contraction and extension are not sensitive to inhibitors of protein synthesis (cycloheximide), or drugs that influence either microtubules (colchicine) or actin-based microfilaments (cytochalasin D). Detergent-extracted cell models contract and extend their flagellar roots and move their nuclei in response to alterations of free calcium levels in the medium. Cycles of nuclear movement in detergent-extracted models require ATP to potentiate the contractile mechanism for subsequent calcium-induced contraction. Flagellar root contraction and nuclear movement in Chlamydomonas may be causally related to signaling of induction of flagellar precursor genes or to the transport of flagellar precursors or their messages to sites of synthesis or assembly near the basal apparatus of the cell.