In the crab,Cancer borealis,initial clearance studies showed a potent renal excretory system for the model organic cation, tetraethylammonium ion (TEA). TEA clearance averaged 145 +/- 32 ml/day, which was 18 times the paired polyethylene glycol clearance. TEA uptake by slices of urinary bladder was concentrative, saturable, inhibitable by N-methylnicotinamide chloride, and dependent on glycolytic, but not oxidative, metabolism. When mounted in flux chambers, bladders exhibited a large net secretory flux. For 0.1 mM TEA, the ration of secretory to reabsorptive fluxes was 65. Urinary bladders from another crab Cancer irroratus,and a lobster, Homarus americanus,also exhibited net TEA secretion. In C. borealisbladder, secretory transport was concentrative, saturable, and nearly abolished by addition of 1 mM quinine to the serosal bath. Reabsorptive transport was not concentrative and was not reduced by luminal quinine. The data are consistent with a secretory pathway that is transcellular and mediated by carriers at both the serosal and luminal membranes.