Previous studies by Kaissling et al. ('75) showed that superficial and midcortical glomeruli of the Psammoinys kidney give rise to short nephrons which do not enter the inner medulla. The thin descending limbs (TDLs) of such short nephrons descend into the inner stripe of the outer medulla only within vascular bundles. The TDLs of long nephrons are excluded from these bundles.Transmission EM was used in the present study to examine the ultrastructure of short TDLs in relation to the arterial and venous vasa recta. In the interbundle region of the uppermost outer medulla, proximal tubules of short nephrons give rise to TDLs classified as having Type ZZ (interbundle type) epithelium. Type ZZ is simplified to Type Z (bundle type) epithelium where the short TDL enters the vascular bundle. It persists throughout the vascular bundle and continues briefly into the interbundle region until its transition to the distal tubule.