BOTH EARLY AUDITORY evoked potentials (BAEPs) and middle-latency auditory evoked potentials (MLAEPs), were recorded in nine patients suffering from a quadrigeminal plate tumor. These recordings were performed before surgery in six cases and after surgery in three cases. The results of these examinations were correlated with impairments of the midbrain auditory pathways as shown by magnetic resonance imaging. BAEPs and MLAEPs were abnormal in five of nine cases and eight of nine cases, respectively. The two examinations yielded normal results in only one case. These data show that the functional evaluation of the midbrain should not be limited to the recording of BAEPs, routinely performed for brain stem functional evaluation, but should also include recording of MLAEPs, although the technique is a little more delicate. In the five patients with abnormal BAEPs, I-V conduction time was increased unilaterally (three patients) and bilaterally (two patients), and the I/V amplitude ratio was abnormal in two patients. In one of these two patients, isolated destruction of the right inferior colliculus was responsible for an abnormality affecting Wave V of the BAEP that was visible only after left ear stimulation. The most frequently observed MLAEP abnormality was a delay in the peaking of the Pa component, assumed to be of cortical origin. Therefore, a limited impairment of the midbrain may delay the peaking latency of Pa. Unilateral hypovoltage of Na-Pa was also observed. In only one case were Na and Pa components unilaterally abolished, in a patient suffering from a postoperative lesion extending from the right inferior colliculus to the right medial geniculate body. Although the generators of the Na component are still a matter of dispute, our recordings show that a midbrain lesion may alter this component.