Pathologic changes related to chiormadinone acetate (CAP) implantation were examined using 14 bitches given doses 2.5 to 25 mg/kg for 2 years. Absence of corpus luteum in bitches given 5 mg/kg or more supported long-term preventive effect of CAP on estrus. The uteri were dose-dependently enlarged and mucometra was occasionally found. Endometrial epithelium hyperplasia was observed but less in smaller doses. Changes in the mammary gland were only growth and lactation at normal degree. No remarkable changes were observed in ACTH and LH cells in the pituitary gland. Low, stable levels of CAP maintained in plasma by subcutaneous implantation seemed to be the main reason for absence or slight CAP-related pathologic changes.