Microtubules at steady state show dynamic instability. This implies that two populations of microtubules exist: a high concentration of slow growers and a small concentration of fast shrinkers. Transitions between the two are possible. The rate constants of these transitions have been derived from the observations of the switching frequency of single microtubules in the microscope.It would be interesting to derive the switching kinetics from a whole population of microtubules. An attempt has been made by Caplowet al. (1988) J. B. C. 263, 10344–10352, however, using a slow temperature perturbation. We have repeated these experiments using a very fast (0.2 millisec) pressure perturbation.Equations have been derived for the decription of the relaxation process of steady state microtubules, taking into account the existence of a growth to shrinkage interconversion. These relations predict a rather small amplitude for the switching relaxation if the T.GTP or T.GDP.Pi cap is small.Experimental results show that the relaxation data can be described by a single exponential. These results indicate that the reshuffling of the growth to shrinkage interconversion does not show up in steady state relaxations, indicating a rather small T.GTP or T.GDP.Pi cap.