Three experiments were conducted to measure changes in plasma thyroxine (T4) concentrations occurring in male and female Japanese quail in response to treatment with dietary thiouracil (TU) and different CP levels prior to sexual maturity and to determine the influence of selection for growth under TU and protein stress on this response. Selected and unselected lines of quail were fed diets containing .2% TU or two levels of CP (20 or 28%) or both from 0 to 4 wk of age. Body weight and plasma T4 were measured at 5, 7, and 9 wk of age in Experiments 1 and 2. In Experiment 3, body weight was measured at 2, 4, 5, 7, and 9 wk and T4 at 4, 7, and 9 wk. Thiouracil inhibited growth to a greater degree than did decreased dietary CP. However, offspring from selected quail were more resistant to dietary TU when selection diets contained TU. When fed as part of a selection regimen rather than to unselected birds, low CP, TU diets inhibited body weight increase to a greater degree and longer after birds were returned to control diets. Thyroxine concentrations between 4 and 9 wk were affected by TU but not by CP level. Thiouracil significantly reduced T4 during treatment; however, T4 was elevated by 3 wk after cessation of TU treatment. Increases in T4 were greater and more immediate in selected than in unselected birds. These findings reveal the ability of quail to compensate for thyroid suppression after TU is removed from the diet and the influence of selection on this compensatory response.