The deleterious effects of decongestant drops on the respiratory nasal mucosa of guinea pigs were studied histopathologically and histochemically. The histopathological changes included: initial goblet cell hyperplasia, squamous metaplasia, increased vascularity, oedema of the corium, mononuclear cellular infiltration and glandular hyperplasia. The histochemical changes indicated: increased secretory activity, increased phagocytic activity as well as disturbance of the vasomotor response.