Summary: Changes in the concentrations of guanosine 3'-diphosphate 5'-diphosphate (ppGpp) in f. sp. were followed during adaptation for growth under denitrifying conditions. The addition of nitrate or nitrite to photosynthetically-grown Palabelled cells resulted in a rapid intracellular accumulation of [P]ppGpp. Coincident with the rise in concentration of the nucleotide there was an abrupt decrease in the rate of [H]uridine incorporation into acid-precipitable cell material. Under these conditions an enhancement of nitrate and nitrite reductase activities was observed. However, washed cells preincubated with 5 m-ascorbate for 60 min did not accumulate ppGpp in response to the addition of nitrate, but the denitrifying enzymes were synthesized. The data indicate an involvement of ppGpp in regulating denitrification, possibly in response to changes in the distribution of available reducing equivalents between photosynthesis and denitrification linked electron transfer when nitrate is included in the growth medium.