Table 5.

Effect of the duration of estradiol plus progesterone treatment on mammary carcinogenesis

Treatment	Mammary cancer incidence	Percent of rats with mammary cancer	Average number cancers per rat
Control	8/9	88	1.6  ±  0.2
30 mg of E plus 60 mg of P, 1 week	1/6	16*	0.2  ±  0.2†
30 mg of E plus 60 mg of P, 2 weeks	0/7	0*	0†
30 mg of E plus 90 mg of P, 1 week	0/8	0*	0†

Rats were treated with MNU at 7 weeks of age. At 9 weeks of age, the rats were treated with E plus P for 1 or 2 weeks. 

^*P < 0.005 compared to control. 

^†P < 0.001 compared to control.