Control analysis of transit time, defined as tau = delta/J, has previously been considered with the constraint of low enzyme concentrations compared with free pools of metabolites [Meléndez-Hevia, Torres, Sicilia and Kacser (1990) Biochem. J. 265, 195-202]. One of the conclusions was that the sum of the control coefficients of the transition time with respect to enzyme concentration was -1. Here we demonstrate that, if the enzyme-bound pools are taken into consideration (which would be important at high enzyme concentrations and high affinities), the sum lies between 0 and -1. The transition time between two steady states, which are frequent physiological events, is mainly governed by time constants involved in changing the enzyme concentrations. Some physiological and evolutionary aspects are discussed.