We present the first results of immunocytochemical (ICC) observations on serotonin (5-HT) and FMRFamide (Phe-Met-Arg-Phe-NH2) immunoreactivity patterns in the rotifer nervous system investigated using a confocal laser scanning microscope (CLSM). Three species of rotifers are studied: Platyias patulus (Plationus patulits, Segers et al., 1993; Hydrobiologia 268: 1-8), Euchlanis dilatata, and Asplanchna herrickii. Independently from their systematic position, these species possess similar nerve structures. However, some differences were observed in the innervation of the corona, mastax, foot, and mostly in the pattern of the cerebral neurons. The general numbers of 5-HT-immunoreactive (IR) and FMRFamide-IR neurons are low (10-34), but constant for each species. The sizes of the neurons vary from 2, 5 to 10 mu m. From 4 to 14 cerebral neurons lie at different levels and are arranged into ail X- or a ring- or a curved arch shape. One or two pairs of neurons are localized along longitudinal nerve cords. Double staining of 5-HT and FMRFamide-IR elements shows no co-localization.