The study shows that the regenerating pharynx ofGirardia tigrinaforms a simple and valuable model system for studies of pattern formation in the nervous system and its relationship to the developing muscles. Immunocytochemistry was used, with antisera raised against 5-HT and the native planarian neuropeptide GYIRFamide. We studied the sequential development of the innervation in the regenerating pharynx, and using TRITC-labelled phalloidin we followed the corresponding differentiation and maturation of pharynx musculature. The peptidergic and aminergic neuronal cell types develop according to different time schedules and different modes. Throughout the process, the GYIRFamide-IR elements are in contact with the old parts of the nervous system, while the apical 5-HT-IR elements developde novo.In the regenerating pharynx the GYIRFamide-IR nerves develop in a proximodistal direction. The first 5-HT-IR cell bodies appear in the tip of the pharynx and are symmetrically placed. They have no contact to the rest of the nervous system. From these cell bodies processes grow disto-proximally and fasciculate with fibres from the proximal part. A striking parallelism between the appearance of GYIRFamide-IR nerves and muscle fibres stained with TRITC-phalloidin was observed. The GYIRFamide-IR nerves cling to the muscle fibres. These results suggest that the contact between muscle fibres and GYIRF-IR nerves is essential for the function of pharynx. The delayed appearance of 5-HT-IR nervous elements is discussed in terms of the influence of 5-HT on sprouting of nerve fibres and synapse formation. The development of the pharynx in tail parts after fission is compared with that after pharynx amputation. The faster rate observed for pharynx regenerating after amputation in comparison with that in regenerating tail parts indicates the importance of the remains of the old nervous system in the pharynx structure.