Antisera against vasopressin, vasotocin, oxytocin, neurophysin-1 and neurophysin-2 were used to investigate immunocytochemically the presence of neurons containing substances antigenically related to these peptides in the nervous system of the Colorado potato beetle. Ten different antisera were used, four against vasopressin, three against oxytocin and one against vasotocin, neurophysin-1, and neurophysin-2. Immunoreactivity was shown by all antisera except those against the neurophysins. The vasopressin antisera all gave different results. One antiserum revealed only a single neuron pair, whereas others revealed in addition one or two other different cell groups. The oxytocin antisera likewise revealed different neurons. The fixation procedure influenced the outcome of the immunocytochemical reaction. Immunoreactivity as revealed by vasopressin, vasotocin and oxytocin antisera is often co-localized in the same neurons; solid phase adsorptions showed that this is due to cross-reactivity of the antisera. Some of the immunoreactive neurons are identical to those recently described to contain a bovine pancreatic polypeptide/FMRFamide-like peptide. This co-localization is probably not due to a cross-reaction. These findings indicate the presence of several vasopressin-like and oxytocin-like substances which in the Colorado potato beetle all have a different degree of immunocytochemical resemblance to vasopressin and oxytocin.