A cytochemical study of p-nitrophenylphosphatase activity in the renal sac of Helix aspersa was carried out, for which the organ was divided in three parts: posterior, middle and anterior. The apical, lateral and basal plasma membranes of the nephrocytes were shown to have both L-cysteine and ouabain sensitive phosphatase activities. The highest level of ouabain inhibition is reached in basal plasma membrane folds of the posterior and anterior zone nephrocytes. The significance of enzyme activity in each of the renal zones is discussed, in order to establish the probable transepithelial route of Na+ in the renal sac of Helix aspersa.