The cat inferior colliculus (IC) was studied with 2-deoxyglucose (2-DG). By presenting high-frequency tone bursts to one ear and white noise bursts simultaneously to the other, a band of reduced or inhibitory labeling was revealed in the central nucleus (ICC) of the IC ipsilateral to the ear receiving the tone bursts. It was concluded that this ipsilateral inhibition might be related to the organization of excitatory/inhibitory units in ICC. In the opposite ICC, narrow bands of increased labeling were seen. In some animals, the positions of single units were marked, and tone frequencies were presented under 2-DG, which were the same as these units' characteristic frequencies (CFs). The positions of the units coincided with the position of the inhibitory bands, indicating that they were functional isofrequency-inhibitory contours. Unlike higher auditory centers, the binaural inhibitory areas were in register with and not orthogonal to the excitatory isofrequency contours. The inhibitory contours were generally larger than the excitatory contours and became even larger in more caudal sections. Both the inhibitory and excitatory contours extended into dorsal cortex areas of IC. In two other cats, high-frequency tone bursts and white noise bursts were presented to the same ear, and both a band of increased and a band of reduced labelling were found in the IC contralateral to this ear. The inhibitory band was always lateral to the excitatory band and was often smaller. They did not become larger in more caudal sections. The position of a unit in one cat was marked by pontamine sky blue, and the position of the unit coincided with the position of the excitatory band. It was concluded that this lateral inhibitory band represents high-frequency inhibitory sidebands of cells with CFs lower than the stimulating tone. It is concluded that the 2-DG method might reveal hitherto unknown inhibitory systems if stimuli could be combined with diffuse stimuli that raised the general background activity of sensory systems.