PtK1 metaphase cells were treated with varying concentrations of nocodazole to reduce spindle microtubule number and spindle length. The range of concentrations employed reduced spindle length from approximately 47% to 82% of the original pole-pole distance. Electron microscopy of cells treated with the lowest concentration of nocodazole employed (0.01 microgram/ml) showed a small decrease in the number of non-kinetochore microtubules (nkMTs), particularly evident in the astral region, with no significant effect on kinetochore microtubule number. Metaphase cells treated with 1 microgram/ml nocodazole for 2 min demonstrated a reduction in spindle length and loss of most non-kinetochore microtubules with little effect on the number and arrangement of the kinetochore class of microtubules. Following nocodazole treatment, the cells were perfused with 0.5 M sucrose dissolved in tissue culture medium, a treatment which has previously been shown to induce spindle elongation in metaphase cells. In cells where nocodazole effected a large decrease in non-kinetochore microtubule number with a concomitant decrease in spindle length, sucrose treatment had a reduced effect in inducing spindle elongation. In cells treated with lower concentrations of nocodazole, where numerous non-kinetochore microtubules remained, sucrose had a greater effect in inducing spindle elongation. These data suggest that the non-kinetochore population of microtubules is responsible for the extent of sucrose-induced spindle elongation. An explanation of these data is provided which suggests that the role of non-kinetochore microtubules is to trap energy in the developing spindle, such that it can be used to separate spindle poles during anaphase B.