Résumé : Administration of insulin for 6 weeks at a daily dose of 2.5 i.u./100 g body weight, together with a 10% glucose solution as drinking fluid, increased tumor growth 8.3-fold as compared with a matched, untreated control group. Administration of insulin alone or of a 10% glucose solution alone produced a smaller yet statistically significant increase (4.8-and 2.2-fold, respectively). In oophorectomized rats, administration of the same dose of insulin, together with the 10% glucose solution for 4 weeks, failed to prevent tumor regression resulting from oophorectomy. On the other hand, in hypophysectomized rats, administration of insulin for 3 weeks at a daily dose of 0.4 to 0.8 i.u./100 g body weight significantly reactivated tumor growth, as compared with a matched control group, when started 21 days after hypophysectomy. Both insulin-treated and control groups received, in addition, a 10% glucose solution and daily s.c. injections of 1.5 mg ovine prolactin; the latter proved by itself incapable of significantly reactivating tumor growth. It is concluded that insulin administered in vivo appears to display intrinsic growth-stimulating properties on the mammary tumor tissue, similar to those previously demonstrated in organ culture. The present study, complementing earlier investigations in alloxan-diabetic rats, provides further evidence suggesting that the rat mammary carcinoma, in addition to being estrogen and prolactin dependent, is also insulin dependent. © 1972, American Association for Cancer Research. All rights reserved.