The Notch gene of Drosophila encodes a single transmembrane receptor that plays a central role in the process of lateral inhibition. This process results in the selection of individual mesodermal and neural precursors during the development of the muscular and nervous systems. The activation of Notch during lateral inhibition is mediated by the transmembrane ligand Delta (Dl) and effected by the transcription factor Suppressor of Hairless (Su(H)). The same functional cassette plays a role in other processes, in particular, the development and patterning of the wing. Genetic analysis has suggested that, in addition to the Su(H)-dependent pathway, Notch can signal in an Su(H)-independent manner. This process seems to be tightly associated with signalling by Wingless, a member of the Wnt family of signalling molecules. Here, we have analyzed further the possibility that the Notch protein encodes two different functions. To do so, we have studied the activities and genetic properties of different Notch receptors bearing deletions of specific regions of the intracellular and the extracellular domains in different developmental processes, and have sought to correlate the activity of these mutant proteins with those of existing mutants in Notch. Our results support the existence of at least two different activities of Notch each of which can be associated with specific structural domains.