Submandibular glands of both sexes from one week to six months-old were serially sectioned, and mucous cells showing a strong positive stain to alcian blue pH 2.5, first detected at two weeks, showed a high degree of inter-sectional and inter-individual variability. In males, the percentage of animals having mucous cells increased with sexual maturation and attained 100 per cent at age six months. In females, the maximum (60 per cent) was reached by four weeks and then decreased with sexual maturation. Only 28.5 per cent of the glands of adult female rats contained mucous cells. In the six-month-old females, the percentage of female rats having mucous cells increased three-fold after injection of testosterone (total dose 17-21 mg; p less than 0.05, Fisher test). Mucous cells in testosterone-injected females displayed morphological features which suggest a transition from original seromucous cells to mucous cells.