Rat pancreas was examined by ultrastructural cytochemical methods for localizing cations and anions, as well as polyanions and, more specifically, sulphated mucosubstance. Exceptionally abundant antimonate-precipitable cation was demonstrated between pancreatic acinar cells and at the base of the centro-acinar and other duct epithelial cells. Precipitates of nuclear heterochromatin appeared lighter whereas those of mitochondria and cytoplasm were coarser and more conspicuous in acinar that duct cells. Stimulation with synthetic secretin at a low level diminished antimonate reactivity of nuclei as well as the precipitation at the basement membrane of centro-acinar cells. At a higher dose, secretin selectively eliminated precipitation between and below centro-acinar and other duct cells while inducing increased antimonate-reactive cation in centro-acinar cells and the acinar lumens. Pancreozymin stimulation elimated antimonate-precipitable cations between acinar cells and, to a much lesser extent, those between duct cells and increased cytoplasmic precipitates on granular reticulum of acinar cells. Silver-precipitable anions were localized on the luminal surface of the apical plasma lemma and the outer surface of the latero-basal plasmalemma of centro-acinar cells but not on acinar cell surfaces. Silver precipitates also occurred on junctional complexes of acinar and duct epithelial cells and at tight junctions of acinar cells and on the inner face of the lateral plasmalemma of acinar cells. Dialysed iron staining demonstrated the most number of sites of acid mucosubstance on the luminal surface of the plasmalemma of acinar cells. Lateral and basal plasmalemmas of centro-acinar and more distal duct cells stained lightly with dialysed iron but those of acinar cells did not. Dialysed iron visualized acid mucosubstance in the lamina lucida of the basement membrane of duct but not of acinar cells. Dialysed iron staining of the plasma membranes succumbed to prior sialidase treatment whereas that of basement membrane resisted digestion. High iron diamine staining demonstrated sulphated mucosubstance in the lamina lucida of the duct basement membrane exclusively. The cytochemical results implicate centro-acinar cells as primarily responsible for contributing fluid and electrolytes to pancreatic secretion.