Ongoing studies in our laboratory, concerned with identifying the neural pathways responsible for the auditory middle latency response (MLR), have involved analysis of surface and intracranial potentials following pharmacologic inactivation (with lidocaine) of small regions in the guinea pig brain. Previous studies indicate that MLR surface waves recorded over the temporal lobe originate from pathways anatomically distinct from those that generate MLR waves recorded over the midline. The medial geniculate body (MG) contributes to both MLR responses. At issue here are the relative contributions of ventral and caudomedial subdivisions, which have been linked to primary and non-primary auditory pathways, respectively. Ventral and caudomedial subdivisions contributed to the surface-recorded MLR in a distinctive manner. Lidocaine injections to both areas reduced the amplitude of the surface temporal response. Caudomedial injections had a much greater effect on the surface midline responses than did injections in the ventral portion. Thus, the ventral division, a part of the primary auditory pathway, contributes chiefly to the temporal response. The caudomedial portion, which may be linked to non-primary auditory pathways, contributes to both responses.