To determine whether the synthesis of secretory proteins changes with age, the incorporation of [14C]-leucine into exportable proteins of the parotid gland was compared in 2- and 24-month-old rats. The proteins were separated by SDS-gel electrophoresis of the post-microsomal supernatant and identified by comparing the banding patterns in gels prepared from unstimulated glands with those from the glands stimulated to secrete. The amount of radioactivity incorporated into the bands corresponding to exportable proteins was significantly less in the older group, indicating that the synthesis of secretory proteins declines with age.