The development of serotonin-immunoreactive neurons in the central nervous system of Xenopus laevis larvae has been studied with special emphasis on the development of the raphe nuclei and raphespinal projections. The first serotonergic neurons were observed in the rostral part of the brain stem at stage 25, only 28 hr after fertilization. By stage 28 some 20 serotonin-immunoreactive neurons were found in the rostral part of the brain stem, bearing small protrusions on the ventromedial side of the soma. These initial axonal outgrowths reach the rostral part of the spinal cord at stage 32. By stage 35/36 the growth cones of the descending serotonergic axons in the spinal cord have reached the level of the anus (10th to 15th myotome). Up to stage 45 the majority of the descending serotonergic axons was found in the dorsolateral part of the marginal zone of the spinal cord. After stage 45 some serotonergic axons were also found scattered over other parts of the spinal marginal zone. Collateral branches were first observed in the caudal part of the brain stem at stage 35/36. Later they occurred also in the rostral (stage 43) and caudal (stage 50) spinal cord, usually on fibers in the ventral half of the spinal cord. The number of serotonergic neurons in the central nervous system (brain stem and hypothalamus) increased steadily throughout development until stage 45. After that the total number of serotonergic neurons in the central nervous system increased about two times faster than the number of serotonergic neurons in the raphe nuclei, due to a massive increase of serotonergic neurons in the hypothalamus. The present study shows that young, just differentiated raphe neurons already contain serotonin. The generation of these neurons appears to take place in the ventricular zone (matrix) of the brain stem between the caudal border of the mesencephalon and the entrance of the nervus octavus. From here these neurons seem to migrate to their final destination. The distribution of serotonin-immunoreactive neurons in the brain stem suggests that a superior (not described so far in Anura) and an inferior raphe nucleus can be distinguished in Xenopus. A rostrocaudal gradient seems to be present in the production of serotonergic neurons which project to the spinal cord. Spinal projections from the raphe nuclei are particularly extensive from the nucleus raphes inferior and gradually decrease rostralwards. In the rostral part of the nucleus raphes superior almost no neurons projecting to the spinal cord are found.