The effect of both isoprenaline (IPN) and pilocarpine (PCP) on mitotic activity and goblet cell number has been studied at 5 levels of the bronchial tree in male and female animals. While both drugs increased mitotic index and goblet cell number at most of the airway levels studied, differences in their effect were detected. After both drugs, nuclei in division were more often superficial within the epithelium than basal but IPN caused a greater increase in mitotic index than PCP. IPN increased goblet cell number at all levels, PCP only in the trachea and larger intrapulmonary airways. The types of goblet cell increased by IPN were particularly those containing acid glycoprotein, whereas PCP increased all types of goblet cell, that is, both those containing neutral and acid glycoprotein. After the drugs, the concentration of cells per unit length of airway epithelium was higher than normal. This is the first study demonstrating the effect of these two drugs on both goblet cell number and cell division. While the two drugs differ in their pharmacological action, the result of their stimulation of airway epithelium is similar. The way in which each effect is produced is discussed.