Antisera against leucokinin I (LK I), a neuropeptide originally isolated from head extracts of the cockroach Leucophaea maderae, were used to localize LK I -immunoreactive (LK I- IR) neurons in ventral ganglia of the sweet potato hornworm, Agrius convolvuli, during postembryonic development. In the larva, prepupa, 5-day-old pupa, and adult, there was no immunoreactivity in the suboesophageal, pro-, meso- and metathoracic ganglia, 1st and 2nd abdominal ganglia (AG), whereas 3 to 7/8 AG were shown to contain LK I- IR neurons. Two pairs of large, and bilateral LK I-IR cell bodies could be found from ventrolateral region within each of 3 to 7/8 AG of the 5th instar larva. AG 7/8 have another pair of weakly-labeled neurons. In the 5th instar larval stage, ventral, unpaired, median (VUM) cells were labeled in the AG 5, 6, 7/8, respectively, and a caudal pair of median LK I-IR cells (MC) also appeared in the 8th AG. However, these kinds of LK I-IR neurons disappeared in the S-day-old pupa. With approaching of the pupa to the adult, bilaterally labeled neurons in the terminal abdominal ganglion (TAG) increased in number from three to four pairs.