1. The effect of long-acting thyroid stimulator (LATS) on the serum luteinizing hormone (LH) levels of the rat in pro-oestrus has been studied.

2. The injection of three out of four LATS-containing immunoglobulin G fractions caused an increase in amounts of serum LH.

3. Adrenalectomy and dexamethasone suppression did not alter this response.

4. Injection of large doses of adrenocorticotrophic hormone did not produce any increase in serum concentrations of LH.

5. It is postulated that LATS may have a direct effect on the release of LH from the pituitary gland.

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