The effects of streptomycin on the synthesis of ribonucleic acid (RNA) and guanosine-3'-diphosphate-5'-diphosphate have been examined in the isogenic strains CP78 (rel(+)) and CP79 (rel-). In the absence of the required amino acids arginine, leucine, or threonine, streptomycin stimulates RNA synthesis in CP78 and this stimulation coincides with cell death. However, in the absence of histidine, also a required amino acid, streptomycin kills the cells without stimulation of RNA synthesis above that which occurs in the absence of streptomycin. In all of these instances, guanosine-3'-diphosphate-5'-diphosphate levels vary inversely with RNA synthesis, decreasing when a stimulation of RNA synthesis occurs. Streptomycin has little effect on guanosine-3'-diphosphate-5'-diphosphate levels in CP78 in the absence of histidine. Another histidine auxotroph, 15 T(-)H(-)U(-) (rel(+)), does show streptomycin-stimulated synthesis of RNA which is coincident with cell death. CP79 (rel(-)) is 10 times more susceptible to streptomycin than CP78 and streptomycin causes an inhibition of the relaxed synthesis of RNA. The greater susceptibility of CP79 may be due to a greater initial uptake of streptomycin by CP79.