1. Determinations of the surface tension of blood plasma from guinea pigs, before and after feeding thyroxin, showed in most cases a definite decrease. Out of twenty-seven experimental animals, twenty showed a depression in the surface tension which ranged from 1.9 to 13.9 dynes, with an average of 5 dynes. Six animals showed variations hardly exceeding the limits of experimental error, while one showed an elevation of 1.1 dynes.
2. Six normal control animals, bled on the same days and kept under identical conditions, showed differences in the two determinations which ranged from a decrease of 0.5 to an increase of 0.6 of a dyne. One other control showed a drop of 1 dyne.
3. Two animals which had been fed large quantities of thyroid extract over a period of 10 days, gave values considerably below normal. One of these animals survived, and after 11 days during which no thyroid extract was given, the surface tension was found again to be within normal limits.
4. The hypothesis is advanced that these changes are due to an increase in the amount of certain normally occurring surface-active constituents, which are produced as a result of increased cellular metabolism.