Pregnant Sprague Dawley rats were fed a tyrosine deficient diet (tyrosine 0.00%): Def. I group started from delivery, Def. II from the 8th day of gestation, and compared with the control (tyrosine 0.36%) until the 5th week of postnatal period. Offspring rats from these groups were analyzed for serum $T_3$ and $T_$. The serum $T_3$ and $T_4$ levels of def. I group were higher than those of control. The serum $T_4$ level of Def. II group were similar to that of the control at 2nd and 3rd week of postnatal period, and then it was rapidly increased to surpass to control level. However, dietary tyrosine deficiency significantly reduced serum tyrosine level at all ages. When considering overall developmental pattern, the serum $T_3$ and $T_4$ levels of all groups increased and serum tyrosine levels increased transiently at 2nd week and then decreased.