The seasonaliy of temperature tolerance using thiourea‐treated, normal and thyroxine treated white perch was determined after laboratory acclimation and LD50 values calculated by normit analysis. Treatment effects, reflected in slight shifts in LD50 values were evident. However, these did not fit a consistent pattern of thyroidal involvement in temperature tolerance.When specimens of known thyroidal functional state were used in tolerance tests, results, similar to the seasonal tolerance tests, were obtained. Thus, although the white perch has a functioning thyroid capable of forming radiohormone and this capability was experimentally varied in these tests, no tolerance shifts were noted.Data obtained in tolerance testing were statistically adjusted to partition out variation due to differences in acclimation temperatures (season) and to test for treatment (thyroidal functional state) effects. Treatment difference was not significant while that due to season was significant (P<0.01). Thyroidal functional state was thus unrelated to toleranc