At the level of the brainstem, precise temporal information is essential for some aspects of binaural processing, while at the level of the cortex, rate and place mechanisms for neural coding seem to predominate. However, we now show that precise timing of steady-state responses to pure tones occurs in the primary auditory cortex (AI). Recordings were made from 163 multi-units in guinea pig AI. All units increased their firing rate in response to pure tones at 100 Hz and 46 (28%) gave sustained responses which were synchronised with the stimulus waveform (phase-locking). The phase-locking units were clustered together in columns. Phase-locking was generally strongest in layers III and IV but was also recorded in layers I, II and V. Good phase-locking was observed over a range of 60-250 Hz: some units (30%) were narrow band while others (37%) were low-pass (33% were not determined). Phase-locking strength was also influenced by sound level: some units showed monotonic increases in strength with level and others were non-monotonic. Ten of the units provided a good temporal representation of the fundamental frequency (270 Hz) of a guinea pig vocalisation (rumble) and may be involved in analysing communication calls.