Gunn rats have a marked deficiency in hepatic UDP-glucuronosyl transferase activity which results in hyperthyroxinemia and hyperbilirubinemia. Their thyroids show a brownish-black discoloration associated ultrastructurally with intracellular dense granules and intraluminal dense masses. In order to determine whether colloid composition and colloid proteolysis are altered in the thyroid of the Gunn rat compared with the Wistar rat, we studied the in situ resistance of thyroid proteins to in vitro proteolysis, the pattern of in vivo (125I) labeled thyroid iodoproteins and the proteolysis of isolated iodoprotein fractions in both strains of rats. For the cytochemical study, thin sections of aldehyde-fixed and plastic-embedded thyroid tissue were treated with 0.3 or 1% pronase in aqueous solution. With the low concentration of pronase, the secretory granules in C-cells and the apical vesicles in follicular cells were extensively digested in both strains of rats, whereas the colloid in the follicular lumen and the colloid droplets were only partially digested. With the high concentration of pronase, the colloid in the lumen and the colloid droplets were more markedly digested in both strains. In the presence of both concentrations of pronase, the dense granules and intraluminal dense masses were unchanged in the Gunn rats. The (125I) iodoprotein pattern was investigated 24 h after a single injection of (125I) iodide and by labeling at the isotopic equilibrium. It was found that the (125I) thyroglobulin fraction was reduced, whereas the (125I) 3-8 S fraction was increased in Gunn rats compared to Wistar rats. Pronase hydrolysis of the soluble (125I) iodine fraction showed similar pronase-resistant fractions in both strains with the single labeling procedure. At the isotopic equilibrium, the pronase resistant fraction was significantly increased in Gunn rats (Gunn 24.0 +/- 5.3%; Wistar: 13.7 +/- 3.1% of the soluble 125I) and a linear correlation was observed between the (125I) 3-8 S fraction of the soluble extract and the pronase-resistant fraction. These data suggest that iodocompounds of small molecular size and low turnover accumulate in the thyroid of the Gunn rat due to their strong resistance to in vivo hydrolysis. A local accumulation of 3-8 S iodocompounds may occur within the intracellular dense granules and intraluminal dense masses in the thyroid of Gunn rat.