Note. —This article and the articles in the previous issues ofThe Journalare part of a series published under the auspices of the Council on Pharmacy and Chemistry. Other articles will appear in succeeding issues. When completed, the series will be published in book form.—Ed. The modern concepts of endocrinology have made it necessary to revise the obsolete humoral pathology of the ancients, which predicated four essential "humors"—blood, phlegm, black bile and yellow bile. Before the presence of hormones circulating in the blood had been demonstrated, the interpretation of many phenomena could be explained only by a humoral hypothesis. Today this hypothesis has been verified. The first direct evidence by a specific test of hormones circulating in the blood was offered simultaneously and independently by Loewe1and his collaborators and by me and my group.2Since then other hormones have likewise been demonstrated in the circulation. This contribution aims