Circulatory levels of triiodothyronine (T3) and thyroxine (T4) and their kinetics were studied in rabbits exposed to intermittent hypobaric hypoxia (5200 m, 395 mm Hg, PO2 83 mm Hg) 6 h daily for 5 weeks in a decompression chamber maintained at room temperature of 22 degrees-24 degrees C. Kinetics of T3 and T4 were studied on days 21 and 28 of hypoxic exposure. The T3 and T4 values were found to be significantly lower on day 8 of exposure to hypoxia compared to the pre-exposure values. The decreased levels were maintained throughout the entire period of hypoxic stress. The metabolic clearance rate, production rate, distribution space and extrathyroidal T3 and T4 pools were significantly decreased in animals under hypoxic stress compared to the control animals. The decline in thyroid hormone levels and their production in rabbits under hypoxic stress indicate an adaptive phenomenon under conditions of low oxygen availability.