Superfused dispersed cells respond rapidly to 2- to 10-min pulses of TRH (10-10 to 10<sup>-7</sup> M) in a dose-dependent manner. The effects of decreasing the stimulus duration can be overcome by a proportional increase in concentration of TRH. A TRH stimulus of 10 min or greater duration results in a sharp peak in TSH secretion followed by a lower plateau. Somatostatin (10<sup>-8</sup> M) inhibits the response to TRH (5 X 10<sup>-9</sup> M). T<sub>3</sub> (2.0 µg/dl) inhibits TRH-induced TSH secretion by superfused pituitary fragments, but not by dispersed cells. Corticosterone (50 µg/dl), however, inhibits crude CRF-induced ACTH secretion by such cells.