In this study, we tried to investigate whether poly-L-arginine (PLA) (MW 10,800) significantly affect mucin release from cultured hamster airway goblet cells and the mucosubstances of hypersecretory air-way goblet cells of rats. Confluent primary hamster tracheal surface epithelial (HTSE) cells were metabolically radiolabeled with $^3$H-glucosamine for 24 hr and chased for 30 min in the presence of varying concentrations of PLA to assess the effects on $^3$H-mucin release. Possible cytotoxicities of PLA were assessed by measuring both Lactate Dehydrogenate (LDH) release and by checking the possible changes on the morphology of HTSE cells during treatment. For in vivo experiment, hyperplasia of rat airway goblet cells and increase in intraepithelial mucosubstances were induced by exposing rats to SO$_2$ for 3 weeks and varying concentrations of PLA were administered inhalationally to assess the effects on the mucosubstances of airway goblet cells of rats. The results were as follows : (1) PLA significantly inhibited mucin release from cultured HTSE cells in a dose-dependent manner; (2) there was no significant release of LDH and no significant change on the morphology of cultured HTSE cells during treatment; (3) PLA also affected the intraepithelial mucosubstances of hypersecretory rats and restored them to the levels of control animals. We conclude that PLA inhibit mucin release from airway goblet cells without significant cytotoxicity and possibly normalize the hypersecretion of airway mucosubstances in vivo. This finding suggests that PLA might function as an airway mucoregulative agent.