The central complex is a prominent neuropil found in the middle of the insect brain. It is considered as a higher center for motor control and information processing. Multiple neuropeptides and neurotransmitters are produced in neurons of the central complex, however, distribution patterns and functional roles of signaling substances in this brain region are poorly known. Thus, this thesis focuses on the distribution of signaling substances and on modulatory roles of neuropeptides in the central complex of Drosophila. Immunocytochemistry in combination with GAL4/UAS technique was used to visualize various signaling substances in the central complex. We revealed different central-complex neurons expressing the neuropeptides; Drosophila tachykinin (DTK), short neuropeptide F (sNPF), myoinhibitory peptide (MIP), allatostatin A, proctolin, SIFamide, neuropeptide F and FMRFamide. Subpopulations of DTK, sNPF and MIP-expressing neurons were found to co-localize a marker for acetylcholine. In addition, five metabotropic neurotransmitter receptors were found to be expressed in distinct patterns. Comparison of receptor/ligand distributions revealed a close match in most of the structures studied. By using a video-tracking assay, peptidergic modulation of locomotor behavior was studied. Different DTK and sNPF-expressing neurons innervating the central complex were revealed to modulate spatial distribution, number of activity-rest phases and activity levels, suggesting circuit dependent modulation. Furthermore, neurosecretory cells in the Drosophila brain that co-express three types of neuropeptides were shown to modulate stress responses to desiccation and starvation. In summary, we have studied two different neuropeptides (DTK and sNPF) expressed in interneuronal circuits and neurosecretory cells of the Drosophila brain in more detail. We found that these neuropeptides display multiple actions as neuromodulators and circulating hormones, and that their actions depend on where they are released. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 3: Manuscript.