A ten amino-acid peptide hormone released by the hypothalamus into the hypothalamus-hypophyseal portal venous system. This used to be known as Leutenizing Hormone Releasing Hormone (LHRH) until it was found to stimulate FSH secretion as well.
This hormone binds to receptors in the adenohypophysis, causing release of LH and FSH.
GnRH levels are primarily modulated via a negative feedback mechanism whereby rising levels of testosterone or estrogen turn synthesis and release of this hormone off. There are, of course, other regulatory factors that affect GnRH levels (you wouldn't think control of your hormones was that simple, would you?)