DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is one of the key enzymes involved in DNA double-strand break (DSB) repair. However, recent studies using DNA-PKcs knockout mice revealed that DNA-PKcs plays an important role in neuronal plasticity. The aim of this study was to examine the role of DNA-PKcs on synaptic plasticity in severe combined immunodeficiency disease (SCID) mice that carry a mutation resulting in a DNA-PKcs protein devoid of kinase activity but still expressed in cells, although with a small COOH-terminal truncation. To this aim, we carried out electrophysiological and molecular analysis on hippocampal slices from wild-type (WT) and SCID mice. Electrophysiological analysis showed an impairment in the basal synaptic transmission in SCID mice compared with WT, whereas paired-pulse facilitation, caused by presynaptic mechanisms, was not different in the two groups of animals. By contrast, tetanic stimulation induced long-term potentiation (LTP) with values that were approximately 43% lower in slices from SCID mice compared with WT. The same slices used for electrophysiology were analyzed to study the phosphorylation state of cAMP response element-binding protein (CREB) and extracellular signal-regulated kinases and to evaluate mRNA expression levels of CREB-target genes at different times after LTP induction. In conclusion, molecular analysis did not show significant differences between SCID and WT brain slices, thus confirming the evidence that DNA-PKcs kinase activity directly regulates neuronal functions and plays a novel role beyond DSB repair. Moreover, these results indicate that studies using SCID mice involving analysis of synaptic function need to be interpreted with caution.