Mexican Rana pipiens now commonly sold in the United States differ morphologically and physiologically from their northern counterparts. They resemble Arizona pipiens in appearance and behavior, but their breeding cycle is distinctive. They are most reproductively active in July and August. During late spring and summer they can be ovulated readily and their pituitary gonadotropin levels are apparently high. Such is not the case during other times of the year. The role of Mexican frogs in teaching and research is discussed.