Food deprivation for 27 and 75h in cocks decreased serum levels of T3 maximally by 61.1% and increased the level of rT3 by 51.4% and that of T4 by 22.4%. Injection of a single dose of TRH (10 micrograms/kg b.w.) increased serum levels of all 3 iodothyronines in both fed and food-deprived animals. In fasted, TRH-treated birds the peak of serum rT3 was 80.5 and 73.3% above control levels in 27- and 75-hr food-deprived cocks, respectively (fed birds: 14.6 and 9.7%); the relevant data for T3 were: 78.3 and 40.4% (fed birds: 95.7 and 127.5%); for T4: 30.0 and 27.0% (fed birds: 9.2 and 6.1%). The greater relative increment of serum rT3 than T3 in food-deprived and TRH-treated cocks was supported by a fall of the serum T3/rT3 ratio to 78.7 and 27.9% in fed cocks and 27- and 75-hr fasted animals, respectively. We conclude, that food deprivation modifies the response of the hypophysis-thyroid axis to exogenous TRH in the sense that during fasting the production of rT3 is enhanced relative to T3.