Tri-iodothyronine (T3)production by interscapular brown fat was studied by measurements of arterio-venous differences and blood flow across the tissue in rats exposed to the following situations: controls, acute cold, chronic cold and starvation. Results demonstrate that brown adipose tissue is a source of systemic T3 in the rat and that the T3 release is modulated according to the physiological situation of the animal: increased in cold exposure and inhibited in starvation.