The plasma and renal clearance of zomepirac, a weak organic acid, was investigated in anesthetized CR Wistar rats after administration of single bolus i.v. injections or continuous i.v. infusions of the 14C-labeled compound. Adjustment of urine pH to the alkaline range caused more than an 8-fold lowering of the plasma elimination half-life (from 7.0 to 0.8 hr) and enhanced renal clearance by a factor of 53 compared to control. Acidification of the urine or probenecid administration increased the elimination half-life (to 10.9 and 17.5 hr, respectively), and decreased renal and plasma clearance of zomepirac. Since zomepirac is highly bound to plasma proteins (approximately 98%), only a small fraction of the drug is available for filtration at the glomerulus. Therefore, the renal elimination of zomepirac is accomplished mainly by active tubular secretion. Passive nonionic reabsorption is a major factor in determining the net clearance of the drug.