Kinetics of accumulation and degradation of ppGpp and pppGpp were analysed in spoT+ and spoT strains of Escherichia coli. The experimental data in this paper indicate that on degradation ppGpp is not converted to pppGpp but instead is converted to GDP which is in turn phosphorylated to GTP. In addition the data are consistent with the idea the pppGpp is a direct precursor of ppGpp. We propose that ppGpp is metabolised according to the following pathway: GTP-pppGpp--ppGpp--GDP--GTP, which we call the ppGpp cycle. Coupled with the observations in spot strains we assume that ppGpp blocks its own synthesis by inhibiting the synthesis of pppGpp but not the interconversion of the two nucleotides.