Persistance in the medial geniculate body (MGB) of spike discharges synchronized with the phase of low frequency tones was investigated in cats anaesthetized with nitrous oxide. Only one out of ten single units responded to tone in a sustained manner. Eighty-eight of these units were analysed with high resolution interval and period histograms. Phase-locked activity was observed up to 1000 Hz, and 20% of the units showing “through” responses below that frequency were phase-locked. In the MGB, phase-locked units have a lower index of synchronization than in the periphery, indicating a higher degree of synaptic jitter at this level. For a given unit, the phase angle shifts linearly with frequency. The slope of these phase-versus-frequency lines is a precise measure of the total transmission delay from the cochlea to the MGB. This delay is a function of the characteristic frequency of the unit, showing that the time spread introduced by the cochlea between high- and low-frequency components of a signal is preserved up to the MGB. [Work supported by the SNSF grant 3.2000.073.]