Rat submandibular and sublingual primordia from fetal day 12 to birth were sectioned serially and stained by various morphologic and histochemical technics to follow development, differentiation and secretory activity. Some findings were: the sublingual begins development later than the submandibular, but its secretory activity starts earlier. Secretory activity of both glands is first detectable on fetal day 17, as trace amounts in submandibular ducts, in larger amounts in sublingual ducts. Secretory activity in terminal cells is first seen on fetal day 18 in the sublingual, and faintly on fetal day 19 in the submandibular. Although the earliest detectable secretory product in both fetal glands is a mucoid type, histochemical procedures indicate that differences exist at this time in the character of the 2 mucous secretions.