Response properties of 142 medial geniculate (MGB) cells were investigated in the awake and undrugged squirrel monkey (Saimiri sciureus). Using Jordan's (1973) parcellation of this complex nucleus, cells were assigned to 3 major subdivisions a, b and c MGB and compared for their general characteristics and response properties. b MBG cells had significantly higher rates of spontaneous firing and longer latency periods than a and c MGB cells. With regard to responsiveness to various auditory stimuli, response patterns, and tuning characteristics, cells in all 3 subdivisions were statistically similar and were thus treated as one cell population. About 95% of the cells responded to broadband white noise, steady tone bursts and frequency modulated (FM) tones. Click activated only 69% of the responding cells. Various "through-stimulus" responses comprised about 80% of the responses. Among the tone-sensitive cells, 90% responded with complex patterns, out of which 50% were frequency-dependent. About 62% of the cells (for which tuning properties were determined) were quite broadly tuned (Q10dB less than 2) and had either single or multi-peaked response areas. The other 38% were quite narrowly tuned (Q10dB greater than 2) and had single-peaked, symmetrical or "tailed" response areas. Different inhibitory and excitatory response components of individual cells had different characteristic frequencies and response thresholds. The c MGB, which is tonotopically organized in a latero-medial orientation, appears to be homologous to the cat pars lateralis of the ventral MGB. The tonotopical organization of the b MGB, which is probably homologous to the cat's medial or magnocellular subdivision, is less clear. Most of the cells which were activated by FM tones disclosed "direction sensitivity" with different degrees of pattern complexity. It is suggested that pitch resolution in the MGB is based on spatio-temporal mechanisms.