In rats, I.V. injected resorcinol led to a decrease in thyroid radioiodine uptake and labelled iodothyronine/iodotyrosine ratio with no changes in the percent of thyroidal labelled iodide or radioiodine release. When thyroid lobes were incubated in vitro with radioiodine, resorcinol concentrations from 10(-3)M to 10(-6)M gave an increase in the percent of thyroidal labelled iodide and iodotyrosine/iodothyronine ratio. In animals fed 5% resorcinol for 2 weeks we observed an increase in thyroid weight, a decrease in plasma T4, no change in the percent of free T4, a decrease in T4 half life and no significant changes in T3 half life. Thyroidal labelled MIT/DIT and T3/T4 atios increased. Resorcinol, given in diet during 5 days to rats which had a low thyroidal thyroglobulin content after PTU administration for 2 weeks, gave, in comparison with rats fed PTU only, an increase in thyroidal thyroglobulin/DNA ratio and a decrease in thyroidal radioiodine release contrasting with an increase in plasma TSH. These differences did not exist when resorcinol was given for one month.