Objective    To investigate the membrane potential characteristics of intracellular responses of individual rat primary auditory cortex neurons to the acoustic stimulation in vivo.     Methods     The intracellular responses of individual primary auditory cortex neurons to the acoustic stimulation in vivo were observed in anesthetized rats using the intracellular microelectrode recording technique.     Results    Sixty-four neurons were recorded in the primary auditory cortex of rats

of which thirty-three responded to the acoustic stimulation with excitatory auditory responses

twenty-four with inhibitory auditory responses

two with  on-off auditory responses

and the remaining five without obvious responses. According to the characteristics of sound-evoked excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) as well as action potential (AP)

the excitatory auditory responses could be classified into 4 patterns: long-term EPSP pattern

short-term EPSP pattern

regular spike pattern and subthreshold EPSP pattern; the inhibitory auditory responses could also be classified into 4 patterns: AP-IPSP pattern

EPSP-IPSP pattern

IPSP pattern and AP-hyperpolarization pattern. The latency [(46.3±20.5)ms] and rising phase duration [(10.1±4.4)ms] of sound-evoked IPSP were significantly longer than those [(15.1±4.7)ms

(6.1±3.5)ms] of EPSP (P0.05

P0.01). The spike intervals and sound durations of  on-off auditory responses were in a phase-locking mode.     Conclusion    Different patterns of auditory responses can be induced in the primary auditory cortex neurons of rats by the same natural acoustic stimulation. Besides

the components and membrane potential characteristics of each pattern are various

which may lay a basis for the functional diversity of primary auditory cortex neurons.