Left upper quadrant (LUQ) cells in Aplysia californica extensively innervate the kidney and regulate some renal functions, although the nature of the neurotransmitters involved in these functions is still unknown. We isolated a new neuropeptide gene (LUQ-1) whose expression in the LUQ cells could be regulated posttranscriptionally by alternative choices of polyadenylation sites. This clone encodes a putative 16.3-kD precursor peptide which contains potential proteolytic cleavage sites that could generate smaller mature peptides. One of these peptides has a 63% identity with mammalian bradykinin/kallidin peptides. The Aplysia haploid genome contains a single copy of the gene, which is interrupted by at least one large intervening sequence. PCR assays performed with RNA isolated from individually dissected cells showed that the LUQ-1 gene is expressed only in neuron L5 among the LUQ cells.