The metlhodls usedl for perftusinig isolatedi renal tul) ules are generally clifferent from microptuncture, as are the prob) lems encounteredl. Altlhotuglh I will concen-trate maiinly on the teclhnical aslpects of perfuision, it seems to me most important to evaluate to wlhat extent the restults are p) ertinent to thestt (ly of kidney function. Are the tubules still" alive?'Are they still fuinctioninig as they did in the aniimal? My answer to the first (qtuestioni is that they are" alive," and I will later suimmarize the evidenice for this 1) y briefly reviewing some tranisport systems in proximal tlubtules. My answer to the secondl questioni is that ftunction slhouldl (liffer in vitro from in vivo even witlh uininjutred tubulessince the environment of the tubtule is alteredl. The tranisport meclhanisms, hiowever, slhotuldbe basically the same, andl the superior control in vitro shouldl allowv individual factors whliicl inflLence tubtule ftunction to be evaluatednmore precisely tlhan in vivo. The most tliffictult part of jn vito perftisioni is dlissection of the ttubules. Despite hiavin (y triecl many otlher animals, we have stuccessftully dissecte (d and perfused tutl) les only from rabbits (l) and (flounders (2). WVe were unsuccessful witlh rats, mice, guinea pigs, hamnsters,(logs, monkeys, frogs, toads, ancl Nectitri. Even witl rabbits tlher-e is variability in the ease of dlissection from animal to animal wlhiclh is tunexplainie (l. Persons learniing the teclhniique or (linar-ily reqtuire several months to dlissect ade (qtiately, andl contiintie to improve witl practice for years. All of the segmenits of the rabbit neplhroni can be dlissected in this fashion except for the papillary collecting Ituct wlhiclh hias mtiltiple branches and tends to split open. The (listal convoluted tubule hias niot been sttudiedsinice it is very slhort (< 1 mm) and (the thin ascendinog limb of Henle's loop is tliffictult to identify. All thle other segments have been tlissecte (l and perfuise (l [proximal convolute (d tuble (3), proxi-mal straiglht ttubule (3), descending limb of Henle's loop (4), tllick ascending limb (Burg, unpublislhed observations), and cortical collecting tubule (5)]. Ttubuiles wlhich have been dlissected are examined microscopically for defects