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The activity of the thyroid glands of Poecilia reticulata was studied by interferometry of the colloid and by histological methods. Interferometric measurements of dry weight per unit area of colloid in sections provided a more sensitive criterion of thyroid activity than histological observation. Active glands from thiourea-treated animals showed a reduction of dry weight per unit area while inactive glands from thyroxine-treated animals showed an increase as compared with control animals. Thyroid glands cultured in a thyrotrophin-containing medium showed a decrease in dry weight per unit area as compared with control glands.

Females during the gestation cycle showed a peak in thyroid activity during the first third of the 24-day cycle. Minimal activity occurred during the second third and intermediate activity during the last third of the cycle at which time the level of activity was much more variable than during the earlier two-thirds of the cycle. Females in which fertilization of the ova had not occurred had inactive although variable thyroid glands, which did not show the cycle of activity seen in the fertilized females. The cycle of thyroid activity is related to the reproductive cycle. It involves a peak in activity at the start of the cycle. The relationships between the thyroid and reproductive systems are discussed.