Changes in the numbers of G cells and the formation and release of gastrin granules have been studies by means of radioimmunoassay and quantitative electron microscopy. The appearance of G-cell granules was affected by the pH and duration of fixation, and after prolonged fixation immature newly formed granules could be identified. In rats fasting up to 3 days first the release and then in turn the maturation and synthesis of granules were depressed. Ultimately the renewal of G cells was inhibited and their numbers declined. During an acute stimulus only a small proportion of total antral gastrin was released and the appearance of G cells was unaltered. In patients treated with cimetidine for 12 months there was no G-cell hyperplasia despite raised stimulated gastrin levels.