Achatin-I and fulicin, isolated from the ganglia and atria of the giant land snail Achatina fulica, are a tetrapeptide and pentapeptide containing a d-Phe and d-Asn at position 2, respectively. We succeeded in cloning a cDNA encoding a precursor of achatin-I from the Achatina ganglia, revealing that the d-Phe present in achatin-I is coded by a common l-Phe codon, TTT or TTC. The deduced polypeptide was found to comprise seven repeats of the achatin sequence GFAD and one analog GFGD flanked on both sides by the typical endoproteolytic site KR. Northern blot analysis of transcripts and Southern blot analysis of reverse transcription (RT)-PCR products demonstrated that achatin-I mRNA was localized in the subesophageal ganglia, whereas expression of fulicin mRNA was detected in the atrium as well as in the subesophageal ganglia. Furthermore, localization of the achatin gene transcript in the right and left pedal ganglia compartments was shown by in situ hybridization on sections of subesophageal ganglia, whereas the fulicin transcript was observed in the right and left parietal ganglia. These data suggested that achatin-I plays an essential role in the regulation of the heart as a neurotransmitter or neurohormone through production in the pedal ganglia and transport to the atrium, whereas fulicin serves not only as a neurotransmitter or neurohormone but also as a novel atrial hormone.