The recent report that purines are competitive inhibitors of specific [3H] diazepam binding to brain membranes has prompted further work concerning the characterization of possible endogenous ligands for the benzodiazepine receptor. In this report, two previously undescribed fractions capable of apparent competitive inhibition of [3H] diazepam binding are reported. Both factors are heat stable and resistant to proteolytic degradation. The larger factor (approximately 700 to 30,000 daltons) is present only in brain and pituitary. The smaller factor (500 to 600 daltons) is found in pituitary, liver, and muscle, but the highest levels are found in brain.