Serotonin increases the frequency and amplitude of spontaneous contractions and leads to an increase in the basal tonus of the locust oviducts. These effects were dose-dependent and were seen on both the non-innnervated and innervated portion of the oviducts. Vertebrate type serotonin agonists and antagonists were used and the profile shows that the receptors on the non-innervated and innervated portion of the oviducts are more similar to 5-HT3 receptors than to either 5-HT1 or 5-HT2 receptors. No serotonin was found associated with the oviducts or the innervation to the oviducts using immunohistochemistry and HPLC coupled to electrochemical detection, suggesting a neurohormonal role for serotonin in the control of locust oviducts. Arch. Insect Biochem. Physiol. 56:179–190, 2004. © 2004 Wiley-Liss, Inc.