We have characterized the effects of vinblastine on the growing and shortening dynamics at opposite ends of individual bovine brain microtubules at steady state in vitro by video microscopy. Vinblastine exerted strikingly different effects on the dynamics and polymer mass at the plus and minus ends of microtubules. At concentrations between 0.1 and 0.4 microM, the drug strongly depolymerized microtubules at minus ends, whereas it did not significantly depolymerize microtubules at plus ends. Vinblastine stabilized plus ends by suppressing the rate and extent of growth and shortening, decreasing the catastrophe frequency, and increasing the rescue frequency. In contrast, vinblastine destabilized minus ends by increasing the catastrophe frequency and decreasing the rescue frequency, whereas it had no effect on the rate or extent of growth or shortening. Thus, vinblastine moderately increased the overall dynamicity at minus ends while strongly suppressing dynamicity at plus ends. Both the kinetic destabilization of microtubules at minus ends and the stabilization at plus ends may contribute to the altered function of mitotic spindle microtubules of cells blocked in mitosis by low concentrations of vinblastine.