Defects in the function and development of GABAergic interneurons have been linked to psychiatric disorders, so preservation of these interneurons in brain slices is important for successful electrophysiological recording in various ex vivo methods. However, it is difficult to maintain the activity and morphology of neurons in slices from mice of >30 days old. Here we evaluated the N-methyl-D-glucamine (NMDG)-based artificial cerebrospinal fluid (aCSF) method for the preservation of interneurons in slices from mice of up to ∼6 months old and discussed the steps that may affect their quality during slicing. We found that the NMDG-aCSF method rescued more cells than sucrose-aCSF and successfully preserved different types of interneurons including parvalbumin- and somatostatin-positive interneurons. In addition, both the chemical and electrical synaptic signaling of interneurons were maintained. These results demonstrate that the NMDG-aCSF method is suitable for the preservation of interneurons, especially in studies of gap junctions.