Table 2.

Effect of synthetic estrogens and progestins on mammary carcinogenesis

Treatment	Duration	Mammary cancer incidence	Rats with mammary cancer (%)	Average number of cancers per rat
Control	3 weeks	9/12	75	1.5 ± 0.3
100 μg eE + 30 mg megesterol	3 weeks	3/13*	23	0.3 ± 0.2*
30 mg tamoxifen + 30 mg progesterone	3 weeks	7/12	58	0.9 ± 0.3
100 μg eE + 30 mg megesterol	1 week	5/13	38	0.4 ± 0.1*
100 μg eE + 30 mg norethindrone	1 week	3/12*	25	0.2 ± 0.1*

Rats were treated with N-methyl-N-nitrosourea at 7 weeks of age. At 9 weeks of age, the rats were treated with hormones for 1 or 3 weeks. Control rats received empty silastic capsules. eE, 17α-ethynyl estradiol. *P < 0.05 compared with controls.