Objective      To investigate the electrophysiological characteristics and underlying mechanisms of neuronal responses to acoustic stimulation in rat primary auditory cortex (A1).       Methods      The late responses of anesthetic rat primary auditory cortex to noise stimulation were obtained by in vivo recording of local field potentials (LFP)

single unit activities (SUA) and intracellular membrane potentials.       Results      We observed sound-evoked LFP with long latencies after the onset responses in A1

which were classified into late responses. The latency of late phase responses (152±18 ms) was significantly longer than the onset latency (9.2±0.4 ms). Single unit recording results further confirmed that the spiking latency (144±11 ms) of late responses was similar to that of LFP. Ten of 29 neurons also showed late responses with intracellular recording. The late depolarization had a long latency (137±11 ms) and duration.       Conclusion      The latency of late responses is longer than that of onset responses in rat A1. The characteristics of late responses (latency and duration) are distinctly different from offset responses. The underlying mechanisms of late responses are probably related to the changed synaptic inputs and/or neuronal excitability.