Taxol has been used as a tool to investigate the relationship between microtubule assembly and guanosine 5'-triphosphate (GTP) hydrolysis. The data support the model previously proposed [Carlier, M.-F., & Pantaloni, D. (1981) Biochemistry 20, 1918] that GTP hydrolysis is not tightly coupled to the polymerization process but takes place as a monomolecular process following polymerization. The results further indicate that the energy liberated by GTP hydrolysis is not responsible for the subsequent blockage of GDP on polymerized tubulin. When tubulin is polymerized in the presence of 10-100 microM taxol, the rapid formation of a large number of very short microtubules (l less than 1 micron) is accompanied by the development of turbidity to a lesser extent than what is observed when the same weight amount of longer microtubules (l = 5 microns) is formed. A slower subsequent turbidity increase corresponds to the length redistribution of these short microtubules into 3-5-fold longer ones without any change in the weight amount of polymer. The evolution of the rate of length redistribution with the concentration of taxol suggests a model within which taxol would bind to dimeric tubulin and to tubulin present at the ends of microtubules with a somewhat 10-fold lower affinity than to polymerized tubulin embedded in the bulk of microtubules. In agreement with this model, binding of taxol to the tubulin-colchicine complex in the dimeric form could be measured from the increase in the GTPase activity of the tubulin-colchicine complex accompanying taxol binding.