Cytophysiological response of the pineal gland to long-term administration of norethisterone (a progestogen-only contraceptive) in rats.

Chronic administration of norethisterone for 75 days caused a marked inhibition of the pineal gland activity of cycling albino rats. In addition to a decrease in the nuclear diameter, possibly the cell size also decreased. The progestin, however, failed to induce mitosis in this gland. It is suggested that prolonged administration of norethisterone exerts untoward effects on the pineal gland function in adult female albino rats.