Physiological gonadotropin levels are modulated by complex relationships between sex steroids and the hypothalamic gonadotropin-releasing hormone (GnRH) pulses. The frequency and amplitude of GnRH secretion provide signals for the differential regulation of LH and FSH secretion. At higher GnRH pulse frequencies, LH secretion increases more than FSH secretion, whereas, at lower frequencies, FSH secretion is favored. Gonadotropin secretion and subunit gene expression are regulated by sex steroids acting either directly at the pituitary level or indirectly by alteration of GnRH pulses from the hypothalamus. And sex steroids have positive or negative actions, depending on the model system and physiological state. Three gonadally derived peptides, inhibin, activin, and follistatin have been isolated and shown to have specific effects on FSH gene expression. Although gonadotropin regulation and reproductive function have not been fully determined, current molecular technologies will probably identify additional targets of sex steroids and GnRH action and allow greater insight.