1. Gastric juice was collected at regular intervals during electrical stimulation of the vagus in anaesthetized cats and during insulin hypoglycaemia in both anaesthetized and conscious cats. The total amounts of acid and pepsin secreted were similar in the three groups.
2. Pepsins were examined by agar-gel electrophoresis. Resting juice contained two pepsins, and up to nine pepsins could be detected after stimulation. Three patterns of pepsin secretion were found.
3. The most noticeable feature was the variation in the proportion of total pepsin attributable to the pepsin which migrated most rapidly during electrophoresis (pepsin 1). In response to insulin hypoglycaemia, anaesthetized cats secreted only a small proportion of total pepsin as pepsin 1 and conscious cats secreted a large proportion as pepsin 1. During direct electrical stimulation of the vagus, the proportion of pepsin 1 rose.
4. The possibility of a dependence of pepsin 1 secretion on vagal stimulation is discussed and the relevance of this to peptic ulcer and to vagotomy is considered.