Variations in thyroxine-binding prealbumin (TBPA) and retinol-binding protein (RBP) concentrations in plasma were determined in Japanese quail subjected to a shortened solar lighting cycle lasting 4 months of real time. The concentration of TBPA varied inversely with the length of daily photoperiod up to 15 h (correlation coefficient r = \m=-\0\m=.\99) when a minimum level was reached of about 50% of that obtained under short days consisting of 8 h light : 16 h darkness (8L : 16D). A slight reversal in the concentration of TBPA occurred with photoperiods in excess of 16 h. The time-lag in response of the cyclic change in TBPA after the change in the lighting cycle was 8 days. The concentration of holoRBP, on the other hand, showed little change with photoperiods <15h, but between 15 and 17\m=.\5h it passed through peak values which were 24% and approximately 10% higher than the initial concentrations in female and male birds respectively. Total immunoreactive RBP concentrations in plasma peaked similarly in females but in males the apparent changes were not significant. The molar ratio of the two proteins (TBPA : RBP) which form a 1:1 complex in plasma also changed inversely with photoperiod from 1 \ m=. \ 6 on short (8L : 16D) to 0\ m=. \ 6 on long (16L : 8D) days. When the ratio declined below 1\m=.\0it was noted that rapid growth of the gonads took place and when this ratio returned to > 1\m=.\0 regression set in. The changes in plasma TBPA concentration were examined in Japanese quail subjected to sudden changes in photoperiod from short (8L : 16D) to long (20L : 4D) days and vice versa. In the group of birds transferred from short to long days the plasma level declined significantly (P<0\m=.\05) within 3 days, whereas birds changed from long to short days were more refractory in that a significant increase was seen only after 30 days. The synthesis of TBPA in liver is closely controlled by photoperiod and it seems that this protein plays an important role in the peripheral distribution of thyroxine and perhaps also influences the cyclical supply of retinol to the gonads.

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