1. Growth hormone (GH)-cDNA was synthesized from poly A(+)-mRNA extracts of chicken pituitary glands. 2. Chicken-cDNA library was cloned into E. coli. 3. Base sequence analysis of chicken GH-cDNA revealed only 70% similarity compared with duck GH-cDNA, and 97% similarity with a previously published chicken GH-cDNA sequence. 4. Dissimilarities in base sequences are primarily observed in the 3'-non-coding region of GH-cDNAs (chicken and duck). 5. Comparisons of amino acid sequences of chicken and duck GH exhibit only three substitutions, while the amino acid sequences of GHs of chicken are identical.