CI-959, 5-methoxy-3-(1-methylethoxy)-N-1H-tetrazol-5-yl-benzo[b]thio phene-2-carboxamide, an anti-inflammatory agent, was considered for development as a treatment for rhinitis. Two-week topical nasal studies in Wistar rats and Beagle dogs were performed to assess nasal toxicity of CI-959. Rats were given daily doses in the right nostril of 0.05 ml of solutions of varying concentrations (0.5, 2, 10, 20, 30, 60, and 90 mg/ml; doses of 0.08, 0.3, 1.6, 3.2, 4.8, 9.6, and 14.6 mg/kg) of CI-959. Beagle dogs were given daily doses in the right nostril of 0.5 ml of 10, 20, 30, 60, and 90 mg/ml solutions (doses of 0.5, 0.8, 1.2, 2.8, and 3.7 mg/kg) of CI-959. Rats given > or = 60 mg/ml either lost weight or had decreased weight gain. Salivation at dosing was seen in both species. Four sections of nasal cavity were examined from each animal. In rats, 0.5 mg/ml was the "no effect" dose; minimal changes were seen at 2 mg/ ml, and significant changes were dose related in severity at > or = 10 mg/ml in all 4 nasal levels. Degeneration and necrosis of respiratory and olfactory epithelia were minimal to moderate in severity. Adhesions and fibro-osseous proliferation of ethmoturbinates, epithelial hyperplasia, squamous metaplasia, and exudate were also seen. In dogs, 10 mg/ml was the no effect dose; respiratory epithelium was affected at > or =20 mg/ml. Respiratory epithelial degeneration was minimal to mild, with loss of ciliated and goblet cells and thinning of mucosa. Distribution of degeneration increased with increased concentrations. In both species, in accordance with the suggested action of CI-959, infiltration with neutrophils was not significant. CI-959 was locally toxic to nasal cavity respiratory and olfactory epithelia in rats and respiratory epithelium in dogs.