A preparatory method, considered to preserve the ultrastructure of the transitional region of lumbosacral dorsal roots in a "useful" way has been tested out. The method consists of vascular perfusion with 5% glutaraldehyde dissolyed in a 300 mOsm Millonig buffer containing 200 mOsm sucrose and 2.7% low molecular dextran, a postfixation in the same medium, a rinse overnight in a normoton phosphate buffer containing 200 mOsm sucrose, an asmication in 2% OsO4 dissolved in a phosphate buffer, an extensive stepwise dehydration starting in 10% acetone and, finally, embedding in Vestopal W.