Observations were made to investigate possible endocrine influences on blood platelets. Adrenal cortex extract failed to influence the platelet counts of mice, rats, or rabbits. Adrenalectomy and sham-adrenalectomy were followed by almost identical platelet increases in mice and rats. Administration of adrenal cortex extract, or physiologic saline, to adrenalectomized rats was followed by a consistent fall in platelets not observed in sham-adrenalectomized rats, or after administering distilled water to adrenalectomized rats. Platelet levels in hypophysectomized rats were significantly lower than in unoperated controls. Splenectomy in hypophysectomized rats was followed by a maximum rise in platelets markedly lower than following splenectomy in intact rats. Bone-marrow megakaryocytes in hypophysectomized rats were significantly fewer than in intact rats. No changes in megakaryocyte number or morphology appeared following splenectomy either in intact or hypophysectomized rats.

ACKNOWLEDGMENT Grateful acknowledgment is made of the frequent advice and aid of Dr. Abraham White.

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