Table 1.

Effect of estriol on mammary carcinogenesis

Treatment	Duration	Mammary cancer incidence	Rats with mammary cancer (%)	Average number of cancers per rat
Control	3 weeks	8/10	80	1.7 ± 0.4
30 mg estriol	1 week	10/12	83	1.6 ± 0.3
200 μg estriol	1 week	11/11	100	1.7 ± 0.3
30 mg estriol + 30 mg progesterone	1 week	7/12	58	1.2 ± 0.3
200 μg estriol + 30 mg progesterone	1 week	9/12	75	1.6 ± 0.3
30 mg estriol	3 weeks	7/11	64	1.0 ± 0.3
200 μg estriol	3 weeks	9/12	75	1.6 ± 0.3
30 mg estriol + 30 mg progesterone	3 weeks	3/12*	25	0.4 ± 0.2*
200 μg estriol + 30 mg progesterone	3 weeks	9/11	82	2.0 ± 0.4

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. *P < 0.05 compared with controls.