Adenohypophyseal fragments from 8 rats were perifused in small 0.2-ml chambers with medium alone or with medium containing 0.2 or 2 µg/dl T3 or 20 µg/dl T4. The TSH secretion in response to 1-min perifusion with 3 × 10–8M TRH was measured before and at 20- to 40-min intervals after beginning T4 or T3 perifusions. A similar temporal course of inhibition of TRH-induced TSH secretion was produced by both iodothyronines, suggesting but not proving that T4 may inhibit the TSH secretion by a direct effect not dependent on its prior intra- or extrapituitary conversion t