Injections of haemolymph from female desert locust, Schistocerca gregaria, which were ovipositing or those that had just finished ovipositing, induced oviposition within hours in females which had ovulated; haemolymph from younger adults failed to do so. Oviposition could not be induced in locusts which had not yet ovulated. Brain extirpation led to failure of oviposition; brain implantation led to eventual oviposition.