The HNK-1 carbohydrate is detectable in perineuronal nets around inhibitory neurons in the hippocampus and neocortex. To address the functional contribution of HNK-1 to interneuron function in the adult brain, we recorded EEG and auditory-evoked potential in freely moving mice deficient for HNK-1 sulfotransferase (ST−/− mice) and in wild-type littermates. While ST−/− mice displayed normal theta oscillations, both cortical and hippocampal oscillations within the beta range were enhanced, and gamma oscillations showed an opposite trend. ST−/− mice had amplitudes of auditory-evoked potentials similar to control mice, but the latencies of their hippocampal responses were shorter. Morphological analysis revealed a decreased density of parvalbumin-positive interneurons in the hippocampal CA3 subfield of ST−/− mice, which may contribute to the observed changes in networks oscillations. These findings reveal alterations in ST−/− mice that differ from EEG abnormalities of mice deficient in the HNK-1 carrier molecule tenascin-R.