Juxtaglomerular neurons (JGNs) of the mammalian olfactory bulb are generated throughout the entire life. Both their integration into the existing neuronal network and their survival depend on sensory activity, but the time point at which adult-born neurons start responding to sensory stimuli is still unknown. To addressed this issue we fluorescently labeled adult-born JGNs by injection of GFP-encoding retrovirus into the rostral migratory stream and monitored sensory-evoked signals in these cells by means of in vivo 2-photon Ca²⁺-imaging. Adult born JGNs started to arrive to the glomerular layer of the dorsal olfactory bulb around 8 days post-injection (DPI). Surprisingly, sensory-evoked Ca²⁺ signals in these cells were observed as early as DPI 9. Already at DPI 9 some JGNs responded to the entire range of odorant concentrations tested (0.1–9% of saturated vapor). In most cases the odorant sensitivity of adult born cells closely resembled the odorant sensitivity of the parent glomerulus and did not differ from that of the surrounding GFP-negative cells. At a given odorant concentration the median amplitude of odorant-evoked Ca²⁺ transients in the adult born cells was 77% (n=10; 1.5% of saturated vapor) compared to the median amplitude of surrounding GFP-negative neurons. At DPI 9–12 62% of adult born cells (n=12) showed spontaneous action potential (AP) firing. Median AP frequency in these cells was 1.9 Hz and thus somewhat lower than that of GFP-positive JGNs at DPI 90 (3.6 Hz, n=8). Adult born cells also responded to the application of an odorant with an increase of action potentials frequency. Taken together our data strongly suggest that adult born JGNs start responding to sensory stimuli shortly after their arrival to the glomerular layer. In terms of response amplitude and odorant-sensitivity their odor-evoked responses closely resemble those recorded from the neighboring mature neurons, implying a remarkably rapid integration of adult-born cells into the glomerular neuronal network.