A controlled study was performed to determine the potential toxic effects of the new nonsteroidal anti-inflammatory drug, eltenac (4-[(2,6-dichlorophenyl) amino]-3-thiopheneacetic acid), in horses. Four treatment groups were formed, each composed of 6 horses. The drug was injected intravenously, once daily, at a dose rate of 0.5 mg/kg, 1.5mg/kg or 2.5 mg/kg for 15 days. A control group was injected with sterile saline solution. Parameters assessed were hay and water consumption, daily clinical observations (evaluation of attitude, mentation, pulse and respiratory rate, fecal consistency, skin condition, and color and hydration of mucous membranes), physical examinations, complete hemogram, coagulation profiles, serum biochemical profiles, urinalysis, gastroscopic examinations and gross post-mortem and histopathological examinations on all organ systems. All examiners were blinded to group assignment and dosage levels until the completion of the study. A few glandular gastric ulcers, mild in severity, developed in seven animals during the treatment period. This occurred more often in horses treated with high doses of eltenac (P=.02). A dose dependent change of WBC count and neutrophil count were noted. Total protein, albumin and globulin levels had dose dependent decreases. One horse in the high dose group (2.5mg/kg) developed ventral edema as well as hypoproteinemia. N one of the horses in any of the dosage groups exhibited depression or anorexia. Gross post-mortem and histologic examination did not reveal any signs of drug related gastrointestinal, renal or hepatic abnormalities. Minimal toxic effects of eltenac given intravenously were greatest in horses treated with 2.5 mg/kg of the compound for 15 days.