The inferior colliculus (IC) receives projections from different sources within the auditory system and processes information ascending from the periphery and to send it forward to cortical structures. The activity pattern of each IC neuron depends on brainstem input and, at least in part, its physiological state and on the types of ionic currents patterning intrinsic discharge patterns. In vitro recording from early stage animals makes the case that voltage-gated K channels underlie the distinct IC neuronals discharge patterns observed (Sivaramakrishnan & Oliver, 2001). However it is clear that maturation of auditory neurons may change channel density (Koch & Grothe, 2003) and auditory-pathway specific K channels may also alter in expression during development (Kharkovets et al. 2000). We have attempted to determine how the presence of voltage-gated K channels contributes IC neuronal firing pattern and synaptic responses in mature animals. 300 µm thick coronal brainstem slices were made from tissue from guinea pigs (250-300g) which included both inferior colliculi. Slices were sectioned in cold ACSF (in mM: 117 NaCl, 4.7 KCl, 2.5 CaCl₂, 1.2 MgCl₂, 25 NaHCO₃, 1.2 NaH₂PO₄, and 11 glucose) and maintained in an incubation chamber at room temperature for 2 h. Slices containing the rostral IC were placed in a recording chamber and superfused with oxygenated ACSF at room temperature. Whole-cell patch-clamp recordings were performed under visual control with a pipette containing in mM: 120 K-gluconate, 5 NaCl, 11 EGTA, 1 CaCl₂, 3 Mg-ATP, and 0.3 GTP-Tris, and 10 HEPES, pH 7.3, 300-310 mOsm). The recordings revealed at least three different types of neurons according to firing patterns and current-voltage relationships. Neurons were in addition stained with Alexa 488 in the patch pipette and their morphology reconstructed using confocal microscopy. The largest group of IC neurons recorded was ‘onset pattern’, firing only once to depolarizing current injection and/or once after a hyperpolarizing pulse. Their resting membrane potential was -57 ± 2 mV and the input resistance 98 ± 10 MΩ (mean± SD, n=8), with prominent outward rectification above -40 mV. Approximately 25% of this outward current was sensitive to perfusion with linopirdine (20 μM) a blocker of the Kv7 family of K channels. A smaller sample of cells although also classified as ‘onset pattern’ exhibited in addition an inward current at negative potentials. This population had a resting membrane potential of 52 ± 2 mV and an input resistance of 55 ± 11 MΩ (n=5). These results show that it is feasible to record properties of adult neuronal cells which underlie the response patterns of cells recorded in the whole animal.