The responses of 256 single neurons in the medial geniculate body of awake squirrel monkeys have been examined with a variety of acoustic stimuli. Both traditional artificial sounds and tape-recorded tokens of species-specific vocalizations were employed, and one goal of the study was the comparison along a number of response parameters of these two classes. Unit responses to the first group (clicks, tone bursts and noise bursts) revealed a level of responsivity approaching 100% in the principal nucleus. Responses to tone and noise were sustained in about half the cells, and characteristic frequencies could be identified in 81%. Responses to vocal stimuli were also very prevalent, approaching 100% in this sample. A comparison of absolute thresholds, response vigor, rate-level functions, binaural interactions, and spectral composition of effective stimuli confirmed that MGB cells process vocal stimuli and artificial stimuli in similar ways. Selectivity of MGB cells to vocal stimuli was quite low, suggesting that processing of communication sounds in MGB is relatively simple and preliminary when compared to auditory cortex.