Pigs were injected i.m. with isoprenaline sulphate on 6 consecutive days. In the bronchial mucosa of animals killed 24 hr after the sixth injection there was a significant increase in the ratio of submucosal gland thickness to bronchial-wall thickness, indicating glandular hypertrophy. The hypertrophy was due to an increase in size of mucous cells, which were filled with secretion. There was also a much higher proportion of acid glycoprotein in the mucous acini of IPN-treated pigs compared with those of controls. This acid glycoprotein was predominantly sialomucin, but a small quantity of sulphomucin, not present in the normal animal, was observed. IPN also caused a significant increase in the number of goblet cells in the bronchial epithelium, but there was no change in their glycoprotein type, most containing exclusively sialomucin. IPN produced a significant increase in the weight of the salivary and adrenal glands, and of the heart, in which the right and left ventricles were hypertrophied equally. Examination of pigs killed up to 3 mth after IPN showed that the submucosal gland hypertrophy persisted for about 1 mth and the goblet cell hyperplasia persisted for 2 mth.