Learning is assumed to be connected with neuronal c-fos expression. In this study we investigate whether associative learning involving tactile stimulation of one row of whiskers induced changes in c-fos expression level in neurons within the cortical representation of the stimulated vibrissae. We use transgenic mice in which the expression of green fl uorescent protein (GFP) is controlled by c-fos promoter [Barth et al. (2004) J Neurosci]. This construct allows us to identify individual neurons undergoing plastic changes in acute (living) brain slices using standard fl uorescence imaging. In the trained mice prior to experiments a 3-day sensory stimulation (20 min/day) of row B of whiskers (conditioned stimulus) paired with an electrical shock (unconditioned stimulus) was performed. The second group comprised of untrained animals. Acute slices from the somatosensory cortex (cut orthogonally to the rows of barrels) were prepared after the end of training and investigated using upright fl uorescent microscope. Our preliminary data indicate high variability of fosGFP expression throughout cortical layers. Biggest amount of GFP-labeled cells are observed in layer 2/3 and much smaller in layers VI and V. Sensory training alter amount of fosGFP+ cells in layer 2/3 of barrel B as compared to other barrels with little or no effect in layers IV and V. Supported by the Ministry of Science and Education grant: N30308131/2682.