The mode of discharge of auditory cortex cells was studied during iontophoretic application of noradrenaline (NA). Only seven of 190 cells showed changes in interspike interval distribution during NA application. A similar conclusion was drawn when the analysis focused on 68 cells classified as burs+ing (n= 15), regular spiking (n= 49) or thin spike (n= 4) cells. Only two bursting cells showed changes in their ISI distribution. The effects on the mode of discharge were independent of the effect on the spike rate and were not a function of cortical depth. These results suggest that the changes in firing mode previously describedin vitrooccur for a limited percentage of cells and/or for cell types not very often recordedin vivo.