The presence of the 29-amino acid peptide galanin and galanin-like immunoreactivity in the peripheral and central nervous system have been established. We describe below the construction of a 1.1-million-recombinant cDNA library derived from poly(A)+ RNA from pig adrenal medulla, rich in galanin-like immunoreactivity, and the isolation and characterization of clones encoding preprogalanin. Mixtures of synthetic oligodeoxyribonucleotides corresponding to galanin-(7-19) and galanin-(22-29) were synthesized and used as hybridization probes to screen the cDNA library. Sequencing of two recombinant plasmids shows that pig galanin mRNA in the adrenal medulla encodes a precursor protein of 123 amino acids that is comprised of a leader sequence, a single galanin sequence, and a 59-amino acid sequence (galanin message-associated peptide) of as yet unknown function. The single galanin sequence, followed by a glycine (the amide donor), is flanked on both sides by pairs of lysine and arginine residues. RNA blot-hybridization analysis of poly(A)+ RNA from pig adrenal medulla reveals a single mRNA of approximately equal to 900-950 bases.