The thyroid glands of the rats treated with 100 mg/kg/day of minocycline for 35 days showed black discoloration. In these thyroids, brown pigment granules were seen in the follicle epithelial cells, and they histochemically stained in the same manner as melanin. By electron microscopy, the deposition of the electron-dense material first occurred in lysosome-like granules and, with further administration, was observed also in the rough endoplasmic reticulum in some cells. Thin-layer chromatography showed that the Rf value of the extract from minocycline-treated rat thyroid was the same as that of the black substance obtained by mixing minocycline and 3% hydrogen peroxide but different from that of melanin. In minocycline-treated rats, the release of thyroxine (T4) from perifused thyroids was significantly less than that from control, and the analysis of iodoamino acids showed an increased monoiodotyrosine fraction. Thus, the pigment of thyroid in minocycline-treated rats was demonstrated to be a metabolic derivative of minocycline itself and minocycline may interfere with thyroid function in high-dose, long-term treatment.