The histochemical characteristics of the epithelial mucins, which are produced by the lining epithelium, the superficial glands and the compound glands of the proventriculus were investigated in the chick embryo from the 7th day of incubation to hatching. Results showed that the time of appearance of the carboxylated, sulphated and neutral mucosubstances differed at the three investigated sites. From the 13th to the 19th day of incubation, intense production of the various mucins was detected at the three sites. However, the modality of intracytoplasmatic storage and the production of mucins in both the cells of the lining epithelium and the superficial glands appeared to differ from those observed in the cells of the compound glands. At the 20th and 21st day, the mucins were primarily produced by the lining epithelium and the superficial glands, whereas production diminished in the compound glands. Furthermore, from the 7th to the 17th day of incubation, the cells of the principal ducts of the compound glands showed the same histochemical characteristics and modality of production as those of the glandular cells of the compound glands. On the other hand, from the 18th day to hatching, these cells assumed the same characteristics as those of the lining epithelium and of the superficial glands.