GTP hydrolysis associated with polymerization is a distinctive feature of microtubule assembly. This reaction may be fundamentally linked to the dynamic properties of microtubules in vivo. Kinetic analysis of the connection between microtubule assembly and associated GTP hydrolysis indicates that these two events are kinetically uncoupled, GTP hydrolysis occurring after tubulin incorporation in the microtubule. As a consequence, the combination of the diffusional incorporation of GTP in microtubules at steady-state and of subsequent GTP hydrolysis results in the formation of a steady-state GTP cap at microtubule ends. The interplay between GTP and GDP at microtubule ends is examined. Inhibition by GDP of steady-state GTP hydrolysis at microtubule ends and of microtubule elongation is understood within a tight reversible binding of GDP at microtubule ends generating 'inactive' elongation sites. Nucleotides are freely exchangeable at microtubule ends. This result indicates that the nature of the nucleotide present at microtubule ends must be considered in a model for microtubule assembly. These data are pooled in order to define the general features of a model describing microtubule assembly and treadmilling in terms somewhat different from previously proposed models.