Mutants in the spo T gene have been isolated as stringent second site revertants of the relC mutation. These show varying degrees of the characteristics associated with the spoT1 gene, viz relative amount and absolute levels of both pppGpp and ppGpp and the decay rate of the latter. The entry of 3H-guanosine into GTP and ppGpr pools in spoT+ and spoT1 cells either growing exponentially or during amino acid starvation was determined, and the rate of ppGpp synthesis and its decay constant calculated. During exponential growth the ppGpp pool is 2-fold higher, its decay constant 10-fold lower, and its synthesis rate 5-fold lower in spoT- than in spoT+ cells; during amino acid starvation the ppGpp pool is 2-fold higher, its decay constant 20-fold lower, and its synthesis rate 10-fold lower in spoT than in spoT+ cells. In one of the "intermediate" spoT mutants the rate of entry of 3H-guanosine into GTP, ppGpp and pppGpp was measured during amino acid starvation. The data form the basis of a model for the interconversion of the guanosine nucleotides in which the flow is: GDP leads to GTP leads to pppGpp leads to ppGpp leads to Y. Calculations of the rates of synthesis and conversion of pppGpp and ppGpp under various conditions in various spoT+ and spoT- strains indicate that the ppGpp concentration indirectly controls the rate of pppGpp synthesis.