TheDrosophilafibroblast growth factor (FGF) receptors Heartless and Breathless are required for the morphogenesis of the mesoderm and the tracheal system. In this article we discuss a number of questions relating to the morphogenesis of these tissues and speculate on poorly understood aspects of the underlying mechanisms. As yet a ligand has not been identified for Heartless, but in the case of Breathless the ligand may in some situations act as a chemotactic signal. We consider it unlikely that release of a distant chemotactic signal plays a role in the morphogenesis of the mesoderm. Instead we propose that the change in the mesoderm from an invaginated epithelium to a single layer of cells spread out on the ectoderm could be a result of the mesodermal cells trying to maximize their contact with the ectoderm. Exactly how the activation of the FGF receptors affects cell behaviour and leads to cell movement is not understood. The signal could simply be permissive, causing cells to become motile, or it could act as a directional signal for cells that are already motile, or perhaps provide both functions. Furthermore, it is unclear how signal transduction is coupled to morphological change. It seems unlikely that activation of transcription targets is essential for cell migration and it is possible that FGF signalling may have a direct effect on the cytoskeleton independent of the activation of the mitogen–activated protein kinase cascade. Analysis of the function ofdof, which encodes a cytoplasmic protein required for FGF signal transduction may provide an insight into these issues.