Significant increases in the concentration of plasma glucagon-like 
immunoreactivity (GLI) and plasma levels of free fatty acids (FFA) and 
triglycerides (TG) concomitant with decreases in circulating levels of thyroxine 
(T4) and triiodothyronine (T3) and T3/T4 ratio were observed in homing pigeons, 
untrained for 3 months, after a flight of 48 km lasting 90-160 min. The 
increased level of FFA is attributed to glucagon stimulated lipolysis. The 
elevation of TG levels may be due to altered partitioning and utilization of 
lipoprotein in adipose tissue and muscle. Reductions in plasma T4, T3 and T3/T4 
ratio are probably due to inhibition of T4 secretion and 5'-monodeiodination 
with possible conversion of T4 to reverse T3 (rT3). These processes may 
represent a mechanism for regulation of thyroid hormone metabolism during 
strenuous and extended flight.