For Escherichia coli B/r growing in glucose minimal medium, the following parameters of RNA synthesis remained invariant between 20 and 40 degrees C: RNA polymerase concentration (RNA polymerase/mass), rRNA and tRNA concentration (RNA/mass), RNA polymerase activity (fraction of total RNA polymerase actively engaged in RNA chain elongation), and stable RNA synthesis relative to total RNA synthesis. The following parameters increased 3.4-fold over the same temperature range: rRNA chain elongation rate, guanosine tetraphosphate (ppGpp) concentration, and culture growth rate. Above 40 degrees C, the changes became more complex, and the growth rate began to decrease. The observation that most RNA synthesis parameters are temperature invariant despite the increase of ppGpp suggests that the mechanism of RNA synthesis control by ppGpp, assumed to involve an interaction of RNA polymerase wtih ppGpp, is itself temperature dependent such that, with increasing temperature, higher concentrations of ppGpp are required to affect the RNA polymerase.