We investigated the firing pattern and frequency tuning properties of medial geniculate body (MGB) neurons, through in vivo intracellular recordings in anesthetized guinea pigs. Twenty-two of the 25 physiological characterized neurons were anatomically identified. Ten neurons were located in the ventral division of the medial geniculate body (MGv) (seven in pars ovoidea (OV) and three in the pars lateralis (LV)). Eight were located in the dorsal division (MGd), and four in the medial division (MGm). OV neurons showed a uniform, phasic ON response with high frequency selectivity. Functionally, they are interpreted as relaying spectral information with high reliability. LV neurons exhibited various patterns: phasic, tonic and excitatory postsynaptic potentials (EPSP) with a spike train. These high magnitude EPSPs are proposed to convey temporal information of the auditory signals with more encoding power. MGd neurons had relatively low best frequencies while MGm neurons had high intensity threshold, broader frequency selectivity, and a tonic response pattern. Tonic firing is likely to impose a strong impact onto wide cortical area and amygdala. When hyperpolarized with current injection, MGB neurons evoked low-threshold calcium spikes. Distinct change in these spike numbers was observed among MGv and MGd neurons as compared with MGm neurons, implying their differential roles. MGm neurons are more modulatory in nature, while the long lasting bursts of low-threshold calcium spikes observed in MGv and MGd neurons probably participate in propagating the sleep oscillations.