Treatment of rats with the beta-agonist isoproterenol causes a dramatic increase in a series of proteins rich in proline (proline-rich proteins) in the parotid glands (Muenzer, J., Bildstein, C., Gleason, M., and Carlson, D. M. (1979) J. Biol. Chem. 254, 5623, 5629). These proteins which contain about 43% proline, comprise more than 50% of the total soluble protein of glands of rats treated with isoproterenol for 10 days. Further studies by in vitro translation analysis using the reticulocyte lysate system and labeling with [3H]proline or [35S]methionine show definitive changes in patterns of protein synthesis and proline-rich protein mRNAs are highly elevated in treated animals. Analysis of translation products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed (1) very little synthesis of proline-rich proteins from poly(A+) RNA of glands of normal rats, (2) poly(A+) RNA from glands of treated animals synthesize mainly proline-rich proteins, (3) translations with [3H]proline and [35S]methionine give identical labeling patterns from cell-free translations are all precipitated by antibodies to proline-rich proteins. At least six different proline-rich proteins are translated with poly(A+) mRNA from glands of treated animals. Each of these proteins is likely a translation product of a separate, specific mRNA. The dramatic changes in protein synthesis of rat parotid glands in response to isoproterenol treatment suggest to us that the parotid gland of the isoproterenol-treated rat is an excellent model system to study the overall responsiveness of gene expression to catecholamines.