The Ras-controlled extracellular signal-regulated kinase (ERK) pathway mediates a large number of cellular events, from proliferation to survival, from synaptic plasticity to memory formation. In order to study the role of the two major ERK isoforms in the brain, ERK1 and ERK2, we have generated GFP fusion proteins of both protein kinases. In addition, we have produced two swapped constructs in which the N-term tail of ERK1, the domain responsible for its unique properties, has either been removed from ERK1 or attached to ERK2. We demonstrated that all four GFP proteins are properly expressed in vitro in mouse embryo fibroblasts. However, only ERK1 and ERK2 > 1 overexpression resulted in a significant growth retardation. In addition, we have expressed all four GFP fusion constructs in vivo, in the adult brain, using lentiviral vector-assisted transgenesis and found that they are predominantly neuronal. Finally, we have devised an ex-vivo system, in which brain slices prepared from adult mice can be stimulated with glutamate and the activation of both cytoplasmic and nuclear substrates of ERK can be detected. Since phosphorylation of both the ribosomal protein S6 and of the histone H3 is completely prevented by a chemical inhibition of the ERK pathway, this ex-vivo system can be exploited in the future to investigate the regulation of the ERK cascade using slices from LV-injected or conventional transgenic animals.