Acidification of endocytic compartments occurs within 2-3 minutes following internalization. In CHO cells, three kinetically and morphologically distinct endocytic or recycling compartments can be identified. Measurements of the pH in these compartments show that the acidity of the compartments is regulated independently, although some parts of the acidification mechanism may be shared. The pH values found in these compartments provide a satisfactory explanation for various required steps in the endocytosis and recycling pathways. Further work is required to understand the biochemical basis for the pH regulation in various intracellular organelles.