The L-Triiodothyronine (L-T3) has a direct influence on the population of somatotrophs in rat pituitary gland. This effect is dose-dependent and induces both proliferation of somatotrophs and striking changes in the synthesis and secretion of growth hormone (GH). Daily injections of 5 micrograms L-T3 for 7 days increased significantly the synthesis and storage of GH in pituitary gland, but the GH release was partially blocked. By contrast, injections of 10 micrograms L-T3 promote rapid synthesis and secretion of GH with removal of the cytoplasmic stores of the hormone and a consequent rise of serum levels. A close correlation was found between levels of stimulation and proliferation or retrogression of lactotroph cell population.