A total of 442 neurons was isolated in the posteromedial thalamus and subthalamus of 20 chloralose-anesthetized cats. The largest samples were obtained from the dorsomedial nucleus (MD) and the centre median/parafascicular complex (CM/Pf) and their border regions. The majority of neurons in MD and in CM/Pf were driven by acoustic stimulation, and many of these cells also responded to visual and/or somatosensory stimulation. The most frequently observed response pattern to noise-burst stimulation was a simple onset response. However, significant proportions of MD neurons were characterized by an onset response followed by some form of later discharge (13%) or by responses of extremely long latency (13%). The overwhelming majority of units in both MD and CM/Pf were characterized by broad frequency tuning, responding over 5-6 octaves or more. Occasional cells exhibited sharp tuning comparable to that in the primary auditory pathway. A large number of cells in each region were relatively insensitive to tones but had low white-noise thresholds; the convergent nature of the input to these cells appears to make them particularly responsive to spectrally complex sounds. Onset-component latencies in MD and CM/Pf ranged from 16 to 60-80 ms, but the distributions in the two areas differed significantly, a larger proportion of CM/Pf neurons having latencies in the 16-30 ms range. The latency distribution for CM/Pf was similar in shape to that obtained previously for the cortical association fields but was displaced by approximately 4 ms toward lower values. The majority of cells in both regions received excitatory input from each ear and were characterized by occlusive binaural interaction. Binaural interaction patterns were stable at all except near-threshold intensities. Differences in response pattern and latency between MD and CM/Pf suggest differences in the organization of their acoustic input. This possibility is discussed in the light of anatomical evidence on the projections to these regions. The identity of acoustic properties in CM/Pf complex and the cortical association fields, and the latency relationship between these regions, provide indirect support for the hypothesis that acoustic input to these areas of cortex is derived from this region of thalamus. However, the relationship of the recently established nonspecific thalamocortical projection system to the physiologically defined association fields remains to be elucidated.