Spinal central pattern generators (CPGs) are responsible for the generation of locomotion characterized by alternation of flexor and extensor muscles and left and right limbs. However, to ensure stable movement, descending input from the brainstem provides on-going excitation to spinal CPGs. For instance, serotonergic descending inputs modulate spinal interneuron excitability. Within the spinal cord, interneurons (INs) can be characterized based on their genetic origins and molecular profiles. One such are V3 INs, having a vital role in regulating balanced motor output. Despite the importance of V3 INs in locomotion, and the importance of descending neuromodulation to initiate and maintain locomotion, it remains unknown how V3 INs respond to descending neuromodulation. We hypothesized that serotonin increases the excitability of ventral V3 INs. Using in-vitro whole cell patch clamp electrophysiology, basic intrinsic and firing properties of V3 INs from thoracolumbar spinal cord slices of Sim1Cre; TdTomato mice were recorded from 3 different age groups (p1-p3, p4-p8 and p9+). Comparisons of intrinsic (rheobase, input resistance, threshold potential and afterhyperpolarization) and firing properties of V3 INs were made between pre-bath and bath application of serotonin. V3 INs demonstrated age-dependent trends where 1/3 of INs had an increase in excitability, another 1/3 had a decrease in their excitability and an additional 1/3 of INs displayed no change in their excitability in response to serotonin(p>0.05). In the future, these findings will lead to better understanding of the differential role of serotonin and its neuromodulatory effects on spinal INs.