Acid extracts of rat SME tissue evoked OAAD in immature rats pre-treated with gonadotropins. Part of the activity in the extracts could be accounted for by their content of LH or vasopressin or both, whereas the remaining activity appeared to be due to release of LH from pituitary of assay rats. The substance(s) responsible for LH-releasing activity of extracts has been called LH-releasing factor. The nature of this material is unknown, but it appears to differ from histamine, serotonin, substance P, epinephrine, and vasopressin or oxytocin.