Ten beluga whales, Delphinapterus leucas, were captured in the Churchill River, Manitoba, held for up to five days, and then released. Blood samples were obtained immediately after capture and at 6-7 h intervals thereafter to monitor changes in circulating levels of thyroid hormones (TH). In six of the whales, total and free thyroxine (T4) and triiodothyronine (T3) declined steadily, whereas reverse-T3 (rT3) showed a transient increase during the first 24-36 h, followed by a decrease to below initial values. The changes in TH may have been due to glucocorticoid-mediated reduction in endogenous thyroid stimulating hormone (TSH), and inhibition of 5'-monodeiodinase in peripheral tissue. Two whales were given 10 IU of bovine TSH immediately after capture, and again one and two days later, resulting in successive increases in all TH, which remained elevated for at least 24 h after the last injection. Thereafter, circulating levels declined as in the untreated whales. Two whales receiving a single TSH injection on the fourth day responded with an increase in plasma TH comparable to that observed following the first TSH injection in the other two animals. Average (+/- SD) circulating level of rT3 at capture was 6.3 +/- 3.1 nmol/L, which is higher than reported for any other mammal and was significantly correlated with the naturally elevated levels of T4 that occur in belugas occupying estuaries during the summer.