Changes in contents of nucleic acids and phosphoprotein of mammary gland of lactating rats resulting from arrest of suckling were investigated. The content of phosphoprotein was used as an index of the amount of milk stored in the gland.
DNA content of the gland began to decrease from 24 hrs. after weaning and diminished to a half of the level before weaning within 3 days. No significant difference was found in the content between 3 and 5 days after weaning. Decrease in RNA content, which began 6 or 12 hrs. after weaning, was much steeper than that in DNA. The ratio of RNA/DNA fell rapidly for 3 days after weaning and thereafter the depression turned to gradual one. Amount of phosphoprotein in the gland, on the other hand, increased abruptly just from the time of litter removal. This increase was continued until 12 hrs. after weaning. Accumulation of the protein was kept in maximum level for about 12 hrs. and then it disappeared rapidly.
The processes of mammary involution resulting from arrest of suckling were classified into 3 phases and the effect of cessation of pituitary hormone secretion and engorgement on each of these phases were discussed.