Rabbit renal cortical slices were incubated at 25 degrees C, pH 7.4, for 2 hr under 100% O2 in a phosphate-buffered medium. [14C]allantoin was accumulated to a tissue water/medium ratio of 1.38 +/- 0.03 (mean +/- S.E., n = 22). Under 100% N2, the tissue water/medium ratio fell significantly to 0.90 +/- 0.02 (n = 14, P less than .001). Probenecid (an anion) inhibited the uptake of allantoin at doses which had no effect on the water content of the slices, i.e., did not appear to have a toxic effect on the tissue. Its effect was specific. In contrast, salicylate (another anion) and quinine (a cation) inhibited the uptake of allantoin only at concentrations which simultaneously increased the water content of the slices. These results suggest a toxic effect on cell metabolism. Salicylate (an anion) and quinine (a cation), in dosage which inhibited allantoin uptake, also inhibit the uptake of the para-aminohippuric acid (an anion) and tetraethylammonium bromide (a cation). Net secretion of allantoin was observed in urinary clearance experiments on anesthetized rabbits acutely loaded with allantoin, under mannitol diuresis. At flow rates less than 1 ml/kg.min, fractional excretion of [14C]allantoin was 0.85 +/- 0.03 (n = 27 clearance periods) indicating significant net reabsorption. When urine flow was increased above 1 ml/kg.min, fractional excretion of [14C]allantoin reached 1.07 +/- 0.03 (n = 18 clearance periods), i.e., net secretion.