Neurons in primary auditory cortex (A1) are known to exhibit a phenomenon known as stimulus specific adaptation (SSA), which means that, when tested with pure tones, they will respond more strongly to a particular frequency if it is presented as a rare, unexpected 'oddball' stimulus, than when the same stimulus forms part of a series of common, 'standard' stimuli. While SSA has occasionally been observed in midbrain neurons which form part of the paraleminscal auditory pathway, it is thought to be weak, rare or non-existent among neurons of the leminscal pathway which provide the main afferent input to A1, so that SSA seen in A1 is likely generated within A1 by local mechanisms. In order to investigate the contributions that neural processing within the different cytoarchitectonic layers of A1 may make to SSA, we recorded local field potentials in A1 of the rat in response to standard and oddball tones, and subjected these to current source density analysis. While our results show that SSA can be observed throughout all layers of A1, right from the earliest part of the response, there are nevertheless significant differences between layers, with SSA becoming significantly stronger as stimulus related activity passes from the main thalamorecipient layers III and IV to layer V.