Summary The activity of ornithine decarboxylase (L-ornithine carboxy-lyase EC 4.1.1.17) and the accumulation of putrescine and spermidine were studied in Escherichia coli CP 78 (rel + ) and CP 79 (rel − ) strains during normal growth and amino acid starvation. Amino acid starvation decreased the activity of ornithine decarboxylase in crude lysates from the stringent strain. Similarly, the accumulation of polyamines was ceased by the deprival of the amino acids in the stringent strain but not in the relaxed one. Partially purified ornithine decarboxylase from Escherichia coli CP 78 was inhibited by guanosine 5′ diphosphate, 3′ diphosphate in a competitive manner with respect to guanosine triphosphate, the activator of the enzyme. These results suggest that the synthesis of polyamines in Escherichia coli CP 78 is regulated, at least partly by guanosine 5′ diphosphate, 3′diphosphate.