Injection of turpentine oil (5 microliter/g, sc) to Sprague Dawley rats was associated with a significant reduction in serum concentration of T4, T3 and TSH that lasted throughout 48 h of study. Dialyzable fraction of T4 and resin uptake of T3 did not change for 10 h after turpentine oil injection, but were increased significantly at 24 h and 48 h. Serum rT3 concentration was decreased significantly at 24 h and 48 h. The pituitary content of TSH in the experimental rat was significantly increased at 24 h after turpentine oil injection, but the TSH-beta subunit content was decreased significantly. Serum TSH response to TRH did not change in experimental rats. Thyroidal radioiodine uptake, serum T4, and serum T3 in experimental rats demonstrated little or no increase in response to exogenous bovine TSH. A thyroid hormone binding inhibitor (THBI) was detected in serum and iodothyronine 5'-monodeiodinating activity was decreased significantly in liver, kidney and heart of rats injected repeatedly with turpentine oil. Oxygen consumption of experimental rats did not change appreciably. Our data suggest that administration of turpentine oil to the rat leads, within a few hours of the injection, to a systemic illness associated with marked changes in thyroidal economy.