A study was made of the optimal conditions for the induction of mammary cancer in the rat. 3-Methylcholanthrene was administered via the gastrointestinal tract, and a simple technique was worked out for inducing mammary cancer regularly and rapidly. Under conducive conditions, which were readily reproduced, multiple mammary carcinomas and these tumors only were induced in every rat in repeated experiments in 60 days or less. In the strain of animal employed in the present experiments, the rapid induction of mammary cancer proved to be a function of (a) dosage, (b) the timing of administration of the aromatic hydrocarbon, and (c) a favoring hormonal status of the recipient.

Most of the established tumors were hormone-dependent because they diminished markedly in size after hormone withdrawal through ovariectomy or hypophysectomy. Similar regression of the tumors was frequently achieved by the administration of dihydrotestosterone. Shrinkage of the cancers was accompanied by atrophic changes. Experimental mammary tumors with these physiologic characteristics have not been recognized hitherto.

The minority of mammary cancers continued to grow after ovariectomy; these are hormone-independent tumors and tumors of this sort had a characteristic cytologic appearance following modification of the endocrine state.

The cell population of a single tumor was not always uniform in its response to appropriate hormonal modifications. In certain tumors in response to changes in the endocrine status of the host many of the cells underwent atrophy whilst other adjacent cells in the same tumor continued to grow so that the net result was a hormone-independent tumor.

Hypophysectomy was the most effective method found to induce regression of mammary cancer in the present experiments.

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