How does the hippocampus know when to store a new memory?We hypothesize that when animals transition into a novel environment,a transient small increase in Dentate Gyrus(DG) activity triggers downstream circuits to establish new neuronal representations.To empirically test this hypothesis,we focus on understanding how neuronal population activity in hippocampal subregions responds to transitions from a familiar(F) to a novel(N) environment. We use an immersive virtual reality setup adapted for rodent head-fixed navigation (n= 6mice).After five consecutive daily training sessions in the familiar environment of 30min each,we observe an increase in performance measured by licking in anticipation of reward delivery(hit-rate:0.21±0.05 hits/lap during the 1st session and 0.98±0.02 hits/lap during the 5th session,p<0.05)reflecting learning of the task.In the sixth session,we introduce the novel environment and compare the performance in both environments.We find that while the performance in the familiar environment increases across sessions, introduction of the novel environment leads to a performance decrease to a level comparable to untrained animals (hit-rate:0.6±0.4 hits/lap in F and 0.2±0.3 hits/laps in N,p<0.05;1st session in F versus N,p>0.05).We record neuronal activity in the hippocampus using Neuropixels probes during the first transition from F-to-N.Based on the anatomical position of the probe,we identify 8 putative DG and 20 putative CA1 principal cells.Their firing frequencies increase slightly in a 1-second time window before and after the F-to-N transition(DG:F,0.5±0.9Hz, versus N,0.8±1.3;CA1:F, 1.46±3.05 Hz, versus N,1.64±3.53).Our experiments will provide a framework for testing the effects of environmental novelty on population dynamics in the hippocampus.