1. One hundred and seventy-six rats of four different inbred strains survived a minimum of 150 days after the implantation of an estrone pellet weighing 8–12 mg. in the subcutaneous tissues of the scapular region.

  2. One hundred and fifty-nine rats of three of the same inbred strains survived a minimum of 150 days after the simultaneous implantation of two similar estrone pellets.

  3. Absorption of the hormone was more rapid from two pellets than from one, and in two of the three strains tested the survival period of the rats with two pellets was only about half as long as for rats of the same strain with one pellet.

  4. Mammary cancers developed in rats of three of the four inbred strains tested with one estrone pellet but not in so high a percentage of the rats as was observed for rats of the same strains treated with a similar dose of diethylstilbestrol.

  5. The minumum and average latent period for the induced mammary cancers was considerably shorter in the rats with two estrone pellets than in rats of the same strain with one estrone pellet. The percentage of rats with induced mammary cancers was increased by the larger dose of estrone in one strain but not in the other because of the shortened survival period.

  6. No mammary cancers were induced in rats of one strain by either dose of estrone. This was the same strain that proved to be resistant to the induction of mammary cancers by diethylstilbestrol.

  7. The induced mammary cancers were classified as 84 papillary cyst adenocarcinomas and four squamous-cell cancers

  8. Bladder calculi and cancer were a frequent finding in rats of the strain that proved to be resistant to the induction of mammary cancer and occurred in a small percentage of the estronetreated rats of the strain with the highest percentage of induced mammary cancer. Adrenal adenomas or cancer were most frequent in rats of the latter strain but occurred in a small percentage of the rats of each strain.

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