Receptor tyrosine kinases (RTKs) and the signaling cascades that they trigger play central roles in diverse developmental processes. We describe the capacity to follow the active state of these signaling pathways in situ. This is achieved by monitoring, with a specific monoclonal antibody, the distribution of the active, dual phosphorylated form of MAP kinase (ERK). A dynamic pattern is observed during embryonic and larval phases of Drosophila development, which can be attributed, to a large extent, to the known RTKs. This specific detection has enabled us to determine the time of receptor activation, visualize gradients and boundaries of activation, and postulate the distribution of active ligands. Since the antibody was raised against the phosphorylated form of a conserved ERK peptide containing the TEY motif, this approach is applicable to a wide spectrum of multicellular organisms.