During early anaphase PtK1 cells were briefly treated with the rapidly reversible microtubule (MT) poison nocodazole. This treatment abruptly stopped chromosome motion and effected a large decrease in spindle birefringence. On removal of the drug, chromosome to pole motion (anaphase A) returned, though at a lesser rate but not extent than untreated cells. In most cases elongation of the pole-pole distance (anaphase B) also occurred, at both a rate and to an extent less than in untreated cells. During the recovery period following drug arrest spindle birefringence did not return to pretreatment levels. Electron microscopic analysis of nocodazole arrested, or arrested and released, cells revealed extensive disassembly of the nonkinetochore class of MTs (nkMTs), particularly evident in the astral region. Microtubules seen in the interzone region were largely fragments of midbody precursors. Kinetochore MTs (kMTs) appeared to be unaffected by the brief drug treatment chosen for these experiments. Analysis of MT profiles seen in transverse sections of the interzone region indicated in treated and released cells approximately 60% fewer MTs. This may suggest that chromosome motion during anaphase is not dependent on interactions between kMTs and nkMTs and separation of the spindle poles can occur in the presence of disrupted interzonal MTs.