Zeranol (beta-resorcylic acid lactone), a common residous component of natural foodstuffs, stimulates developmental growth of the mouse mammary gland.

ICR mice ovariectomized at 6 weeks of age were divided into 5 groups and injected s.c. daily for 20 days with: (1) 0.9% NaCl solution containing 0.1 mg/ml gum arabic (controls); (2) zeranol (Z) (1.0 microgram/day); (3) progesterone (P) (1.0 mg/day); (4) Z + P or (5) 17 beta-estradiol (1.0 microgram/day) + P (E + P). Mammary gland (no. 4, inguinal) growth was assessed by analysis of mammae wet weight, dry fat free weight, DNA content and mammae whole mount evaluation. Administration of Z, alone or in combination with P, increased all 4 measures of mammary growth (P less than 0.05). The combination of Z + P was highly mammogenic, with mammary wet weight, dry weight and DNA content comparable to that obtained with E + P. Mammae development scores of Z + P treated mice were only slightly less than those from E + P treated mice. The mammary glands of Z treated mice had increased duct number (branching) and thickening. The glands of Z + P treated mice had extensive duct branching with occasional lobule-alveolar development. These data indicate that Z, alone or in combination with P, is an effective mammogen possessing mammogenic properties similar to the potent natural estrogen 17 beta-estradiol.