Mitotic PtK1 cells were treated both during mid-anaphase and at furrow initiation with the potent microtubule (MT) stabilizing agent, taxol, to determine the role of MTs in the rate of cytokinetic events. Rates of cytokinesis (micron/min) were measured by changes in furrow diameter. Incubation of PtK1 cells during mid-anaphase with 5 micrograms/ml taxol slows the rate of cytokinesis by an average of 43%. Instead of furrow initiation to midbody formation taking an average of 10.7 min (1.6 microns/min), furrowing to midbody formation was completed in an average of 19.0 min (0.9 micron/min), which does not include the 7-min period between taxol application in mid-anaphase and furrow initiation. Application of 5 micrograms/ml taxol to cells at furrow initiation had a reduced effect on decreasing the rate of cytokinesis and midbody formation; furrowing to midbody formation took an average of 14.6 min (1.2 microns/min). These data suggest that delays in the rate of cytokinesis is dependent on the mitotic stage at which taxol is applied. Ultrastructural analysis shows that taxol treatment of anaphase cells prevents midbody formation during early G1, yet MT number and organization in the furrowed region is not significantly altered from untreated cells. There is little change in the organization and amount of contractile ring microfilaments, yet filaments are also found parallel to midbody MTs. Our results may be explained by the fact that taxol tends to stabilize MTs which probably affects the rate at which they depolymerize in the terminal phases of cytokinesis. Reduction in depolymerization rates of a stable population of MTs could serve to regulate the rate of cytokinesis.