Mammary tumour development in BR6 mice: ovarian influences and 5-hydroxytryptamine.

Pregnancy-dependent mammary tumours in BR6 mice first appeared during the third week of pregnancy. Regressed tumours reappeared chiefly during the second week. Alkaline Phosphatase activity in normal mammary tissue indicated a uniform increase in growth during pregnancy with no marked increase at any particular time. Pseudopregnancy was not sufficient to induce new tumours, though it had some stimulating effect on existing, regressed ones. 5-Hydroxy-tryptamine did not cause tumours in virgin mice, but seemed to influence tumour appearance in breeding females. Its possible mode of action is discussed.