The concentrations of sodium, potassium, calcium, chloride, bicarbonate, and protein and non-protein nitrogen were determined in series of samples of pancreatic juice secreted by dogs in response to (a) constant rate of administration of secretin, (b) varied rate of administration of secretin, and (c) interrupted administration of secretin (rest period, 2 hr.). Spectroscopic as well as chemical methods were used in the analysis of the samples.The data indicate that the membranes of the pancreatic gland offer little resistance to the passage of simple inorganic ions. The compensatory relation between the bicarbonate and chloride concentrations of the secretion is interpreted on the basis of (a) formation of at least part of the bicarbonate within the secretory cells, and (b) the action of membrane forces, probably of an electrical nature. Absorption spectrum studies indicate either (a) that only one type of absorbing protein is present in the secretion, or (b) that if several types are present, they are always secreted in constant proportions whether they are enzymatically active or not. The interpretation of the protein nitrogen and certain other data are given in a subsequent paper.