The Bidder's organ is an undeveloped ovary located anterior to the testis in male true toads (Bufonidae). The presence of sperm and oocytes, derived from the primordial germ cells of the male toad, provides an exceptional condition for the study of germ cell differentiation in vertebrates. In this study, the effects of orchidectomy on morphology and on lamina-associated polypeptide 2 (LAP2) expression were investigated in the Bidder's organ of Bufo marinus. To characterize bidderian oocytes, oogenesis in the ovary was divided into six stages. It was found that ovarian and bidderian oocytes were morphologically identical. To determine the expression of LAP2 isoforms, oocytes from the ovary and the Bidder's organ were examined by sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting. In amphibians, LAP2beta is expressed in somatic cells, whereas LAP2omega is expressed in oocytes. The Bidder's organ normally contained previtellogenic oocytes of less than 150 micro m in diameter that expressed LAP2beta. However, the organ in some male toads contained a few larger oocytes, resulting in the faint detection of LAP2omega. After orchidectomy, bidderian oocytes grew and strongly expressed LAP2omega. Moreover, as in the ovary, LAP2omega was upregulated in bidderian oocytes of 240 microm in diameter. This work represents the first demonstration of the molecular similarity between ovarian and bidderian oocytes.