The uptake of cyclic analogues of probenecid by kidney cortical slices has been studied in detail, in order to obtain more information on the secretory system for these compounds. Both p-piperidyl sulfamyl benzoic acid and p-benzyl sulfamyl benzoic acid were accumulated against concentration gradient, by renal tissue under aerobic as well as anerobic conditions. PAH, phenol red and probenecid competitively inhibited the active accumulation of these compounds by kidney tissue. Aerobic uptake of probenecid analogues was stimulated by succinate and octanoate at low medium concentrations while inhibition of renal accumulation of these compounds occurred at higher concentrations. Both p-piperidyl and p-benzyl sulfamyl benzoic acids like probenecid strongly interact with kidney cortex homogenates. Binding of these cyclic analogues to various cellular constituents of homogenate was efficiently inhibited by probenecid. The binding affinity of probenecid and analogues for kidney tissue, phospholipid vesicles (liposomes) and human serum albumin increased in the order : p-piperidyl sulfamyl benzoic acid less than p-benzyl sulfamyl benzoic acid less than di-n-propyl sulfamyl benzoic acid (probenecid). By contrast to the view put forward by Beyer (1950 & 1954), the results presented in this paper established that probenecid analogues are the true substrates of renal organic anion transport system.