Cloned cDNA sequences for human preangiotensinogen have been isolated from a human liver cDNA library by hybridization with a restriction fragment derived from a previously cloned cDNA for rat preangiotensinogen. Analyses by nucleotide sequence determination, S1 nuclease mapping, and RNA blot hybridization indicate that human preangiotensinogen is encoded by two mRNAs that differ only in the length of the 3'-untranslated region. The deduced amino acid sequence shows that the mature angiotensinogen consists of 452 amino acid residues with the angiotensin sequence at its amino-terminal portion. Two potential initiation sites have been discussed. These are the methionine codon located at the position exactly corresponding to the initiation site of rat preangiotensinogen mRNA and an additional methionine codon positioned nearest the 5' end of the mRNA. The amino acid sequences starting at either of the initiation sites and preceding the angiotensin sequence constitute a large number of hydrophobic amino acid residues, thus representing the signal peptide characteristic of the secretory proteins. Human and rat preangiotensinogens show that 63.6% of the amino acid positions of the two proteins are identical. However, the amino-terminal portions directly distal to angiotensin I diverge markedly between the two proteins and differ in their possible glycosylation sites. These structural differences may contribute to the known species specificity exhibited by renin.