Photoreceptors undergoing target selection in the optic lobe of Drosophila express a nitric oxide sensitive soluble guanylate cyclase (sGC). At the same time, cells in the target region of the optic lobe express nitric oxide synthase (NOS). Pharmacological inhibition of NOS, NO or sGC leads to disruption of the retinal projection pattern in vitro, and the extension of individual retinal axons beyond their appropriate targets. The disruptive effects of NOS inhibition in vitro are prevented by adding a cGMP analog. Mutations in the sGC alpha subunit gene, Gcα1, reduce sGC expression and attenuate NO-sensitive retinal cGMP production in the visual system. Although the retinal projection pattern is undisturbed in Gcα1 mutants, they lack positive phototaxis as adults, suggesting inappropriate connections exist between the photoreceptors and optic lobe interneurons in these flies. Preliminary results show that heat-shock expression of wild-type Gcα1 during metamorphosis can restore positive phototaxis in severe Gcα1 mutants. These in vivo results support the in vitro findings that NOS and sGC activity are required to promote the appropriate retinal innervation of the optic lobe.