Weanling female Sprague-Dawley rats were given either 2 or 5 consecutive daily doses of 30 mg/kg 2,2′,4,4′-tetrachlorobiphenyl (CB 47) or a total dose of 120 mg/kg Aroclor 1242 divided into 2, 3, or 5 daily doses by intraperitoneal injection. One day after the final dose, serum total thyroxine (T4) was determined and thyroid glands were collected and prepared for morphometry analysis. Serum T4increased between 20 and 25 d of age, but declined to 35–52% of controls by d 25 in PCB-treated rats. In rats receiving only 2 doses of CB 47, the declines in serum T4were more modest but the thyroid follicular epithelial cell height increased from 9 µm to 10–12 µm and the colloid area decreased from 1100 µm2to 800–900 µm2. In Aroclor 1242-treated rats, follicular cell height increases and colloid area decreases were somewhat greater; serum T4was higher (partially restored) in the rats having received earlier doses. The rapid response of the thyroid gland to moderate decreases in serum T4attenuates the observed decrease in T4and may mask effects on T4metabolism in short-term structure-activity studies. Morphometric measurements may be helpful in characterizing early and/or transient responses to toxicants such as PCBs that have multiple endocrine disrupting effects.