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In Drosophila melanogaster, eyeless and twin of eyeless have important function during eye development. Mutants of both genes give a variety of eye and head phenotypes with the strongest being almost headless meaning that they lack all structures derived from the eye-antennal disc. toy the first eye specification gene expressed in the regulatory network that leads to eye formation and the gene actvates eyeless. What regulates toy is still not clear and has been the focus of this thesis. In Paper I, we analysed part of the upstream region of the toy gene to be able to drive reporter genes with the expected expression pattern in embyo and larval tissues. We found that a 1300-bp region surrounding the toy transcription start is important for correct Toy expression during embryonic and larval development. We also tested for possible redundancy between the toy and ey genes by rescue experiments on some lethal allels in both genes and found that Pax-6 proteins can substitute for each other concerning both head structures and survival. However, rescue is only partial, indicating that the proteins are not fully compatible or that the levels of expression are not sufficiently reproduced by the artificial Gal4-UAS system. Furthermore, we show that inhibition of apoptosis increased survival in strong toy mutants, but did not improve eye phenotypes. In Paper II, we searched for possibly upstream regulators of toy and found that the head gap gene empty spiracles changed the expression pattern of Toy significantly in the embryonic eye-antennal primordium. By clonal analysis and ectopic expressions, we made the conclusion that Ems acts as a repressor of toy during late embryonic development and also at later developmental stages. In Paper III, we investigate presumptive toy enhancer regions within the intron sequences of the gene. Generation and examination of transgenic lines showed that there might be an enhancer region driving toy expression in the embryonic ventral nerve cord within intron 2.