Temperature-jump studies were made of the interaction of carbamyl phosphate with aspartate transcarbamylase of Escherichia coli by observing the pH changes accompanying binding with a pH indicator. In the absence of aspartate analogs a single relaxation process (Τ = 0.1-0.5 msec) could be associated with the bimolecular binding of carbamyl phosphate to native aspartate transcarbamylase. In the presence of 10 mM succinate, two relaxation processes were observed to accompany the binding of carbamyl phosphate to the native enzyme. The faster process (Τ ~ 0.2-1 msec) could be quantitatively analyzed in terms of an ordered bimolecular binding of carbamyl phosphate and succinate, followed by an isomeri-zation of the enzyme-carbamyl phosphate-succinate complex. The rate constants characterizing the isomerization are similar to those found for the succinate-induced conformational change of the carbamyl phosphate-catalytic subunit complex reported previously. These two conformational changes are probably identical in nature and of importance in the catalytic mechanism. The slower relaxation process (Τ ~ 25-50 msec) has a concentration dependence which is consistent with the allosteric control mechanism of Monod et al. When BrCTP, an allosteric inhibitor, was added three relaxation processes were observed: one associated with the conformational transition induced by BrCTP in the presence of substrates, and two associated with the processes induced by carbamyl phosphate with 10 mM succinate in the absence of BrCTP. The BrCTP has no effect on the faster relaxation process, but reduced the rate of the slower conformational transition. This confirms the interpretation suggested above, namely that the faster relaxation process is associated with the catalytic mechanism and the slower process with the control process. On the basis of these and other results a multiconforma-tional model for regulation is proposed.