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The changes in the kinetics of ¹³¹I131I^– accumulation accompanying the thyrotrophin (TSH)-induced decrease in the tissue/medium concentration ratio (T*/M*) in propylthiouracil-blocked guinea-pig thyroid slices in vitro have been investigated. ¹³¹I^– was eluted in serial 2 ml fractions from prelabelled 10 mg thyroid slices incubated for 10 h with TSH. Retrapping of ¹³¹I^– from the extrafollicular space was abolished by including ClO₄^– in the elution medium. In the presence of ClO₄^–, departures from 1st order kinetics were detected by the analysis of regression of successive estimates of instantaneous exit rate, though not by the analysis of regression of log_eT_t*/T₀*. It was predicted that initial exit data would be more representative of exit rates under equilibrium conditions. TSH gave increases in the fractional exit rate (K_TM) of similar order to those required to account for the decrease in T*/M*.

The data support the hypothesis that an increase in ¹³¹I^– exit rate mediates the decrease in T*/M* induced by TSH.