The neck cells (or mucous neck cells) present in the neck region and the zymogenic cells (or chief cells) present in the base region of the units in the mouse corpus were examined in the electron microscope (EM) and in radioautographs prepared after administration of 3H-thymidine by single or multiple injections or by continuous infusion for 1-52 days. For these studies, the neck region of the units has been subdivided into three equal segments, respectively named high neck, mid neck, and low neck, while the base region has been similarly subdivided into high base, mid base, and low base. The neck region includes an average of 12.6 neck cells, characterized in the EM by dark, mucous secretory granules that frequently exhibit a light, pepsinogenic core. Continuous 3H-thymidine infusion reveals that neck cells come from pre-neck cells, which are believed to arise in the isthmus region from the undifferentiated granule-free cells through a pre-neck cell precursor stage. The pre-neck cells, characterized by the presence of a few cored secretory granules, migrate inward (i.e., in the direction of the blind end of the units) and enter the neck region to become neck cells. It is estimated that 59% of the neck cells arise from differentiation of pre-neck cells, whereas the other 41% are derived from their own mitoses. Neck cells migrate inward in 1-2 weeks from the high through the mid and low neck segments, while they keep on producing more and larger secretory granules and thus further differentiate as mucus-producing cells. When neck cells reach the high base segment, they become pre-zymogenic cells that produce secretory granules in which appear light, irregular, pepsinogenic patches which encroach on the dark mucous content. With time, the pre-zymogenic cells, of which there are 5.0 per unit on the average, keep on producing new granules with larger and larger light patches, so that in the end the cells produce granules which are entirely filled by light, pepsinogenic material. At this stage, the cells are zymogenic cells. Zymogenic cells, which average 67.5 per unit, further migrate inward, while gradually enlarging and producing pepsinogenic granules of increasing size. In the low base segment, some zymogenic cells show signs of degeneration leading to death by either necrosis or apoptosis.(ABSTRACT TRUNCATED AT 400 WORDS)