In the nervous system of adult Schistosoma mansoni, neuropeptides and serotonin were demonstrated immunocytochemically. Neurons and nerve fibers immunoreactive to anti-FMRF-amide, anti-substance P, anti-leu-enkephalin, and anti-growth hormone releasing factor were found in the central and peripheral nervous system. The peptidergic nerve cell bodies in the bilobed brain are large and often multipolar; those in the peripheral nerve net are smaller and often bipolar. Sensory receptors of two morphologically different types were found along the surface of the worm. They are mainly immunoreactive to anti-substance P and anti-leu-enkephalin, and, to a lesser degree, to anti-FMRF-amide. Serotonin-immunoreactive fibers are found in the central and peripheral nervous elements as well as in sensory endings.