Tissues of 50 small cell lung carcinomas were examined for production of 17 peptide hormones. Only when the concentration of a peptide detected in the tumor was 10 pmol or more per g wet weight, was the peptide considered to be produced by the tumor. The frequency of production of at least one of these peptide hormones was 84%, and that of two or more hormones was 50%. These results indicate that peptide hormone production is a very common phenomenon in small cell lung carcinoma. Of the peptide hormones examined, gastrin-releasing peptide is produced with the highest frequency, suggesting that this peptide could play an important role in small cell lung carcinoma.