Manduca sexta allatotropin (Mas-AT) was isolated and first characterized as a peptide that stimulated juvenile hormone biosynthesis in adult lepidopteran corpora allata and was subsequently shown to have cardioacceleratory activity in the pharate adult. In this study, we identified the cells in the nervous system of the insect that contain mRNA encoding Mas-AT and immunoreactivity against a polyclonal antiserum to Mas-AT. In larvae, Mas-AT mRNA and immunoreactivity was most abundant in two cells in the frontal ganglion, which project their axons down the recurrent nerve toward the gut, and in cells in the terminal abdominal ganglion. Lower levels of Mas-AT mRNA were detected in the brain and subesophageal ganglion. In the pupal and pharate adult stages, we detected Mas-AT mRNA and immunoreactivity in cells of the abdominal ganglia and in additional cells in the terminal abdominal ganglion. These additional cells in the ventral nerve cord that express Mas-AT during the pupal and pharate adult stages include cells that differentiate during metamorphosis as well as cells that exist in larvae but do not begin to express Mas-AT until these later developmental stages. Some of the cells that exhibit Mas-AT immunoreactivity lack Mas-AT mRNA, suggesting that the antisera used in this and previous studies recognizes other peptides in addition to Mas-AT. This pattern of expression suggests that Mas-AT may mediate multiple physiological functions during the life cycle of the insect, including the larval stage in which no function has yet been described for the peptide.