The valyl-transfer ribonucleic acid (tRNA) synthetase of Escherichia coli strain NP2907, previously described as having an elevated Km for adenosine triphosphate and reduced stability in vitro compared to the wild type, was found to be conditionally thermolabile in vivo. The rate of inactivation of this enzyme at a particular temperature appears to be coordinated with the rate of growth; at 40 C this coordination results in equal rates of synthesis and destruction over a wide range of growth rates. In vitro studies showed that conditions favoring maintenance of the valyl-tRNA synthetase-valyl adenylate complex conferred complete protection against inactivation at 40 C, whereas the further addition of uncharged tRNA caused rapid, irreversible decay. We propose that the rate of inactivation of this mutant valyl-tRNA synthetase in vivo is a function of the ratio of deacylated to acylated tRNAval and that this ratio is a function of growth rate. The event which renders the valyl-tRNA synthetase susceptible to inactivation is likely to be the normal breakdown of the valyl-tRNA synthetase–valyl-adenylate complex during a cycle of aminoacylation of tRNAval.