One oral dose of pyrazole caused necrosis of rat thyroid follicular epithelial cells but spared the parafollicular (C) cells and the parathyroid glands. Serum thyroxine (T4) and triiodothyronine (T3) were significantly decreased on day 3 after pyrazole administration and were immeasurable on day 5. At day 5 the thyroid was enlarged and the concentration of thyroid-stimulating hormone in the serum was increased, indicating an appropriate pituitary response to a primary lesion in the thyroid. Doses of pyrazole which produced no morphologic change in the thyroids also significantly depressed the concentrations of T4 and T3 in the serum.