Current concepts as to how reabsorption of filtered fluid occurs in the kidney are discussed. Emphasis is placed on certain unique problems arising from the remarkable covariance of amounts of filtered and reabsorbed. A concept advanced by Carl Ludwig over 100 years ago is exhumed and found to provide a reasonable basis for glomerulotubular balance. Some of the difficulties in accepting this view of the renal modus operandi in the light of the current store of information is discussed. It is proposed that a strong a priori case be made for the proposition that a large or at least significant, fraction of tubular reabsorption may occur in the simple manner suggested by Ludwig. Finally a tentative outline incorporating this concept is offered as a reasonable alternative to the theory which is widely held at the present time.