The ultrastructure of rat gastric parietal cells was studied at six timepoints of the 24-hour day. The rats, maintained on a 12 hour:12 hour light-dark regimen, had been subjected to either a 40-hour fast or to a 4-hour mid-light restricted feeding period. At each time point, the volume density (Vv) of secretory canaliculi, surface density (Sv) of microvesicles and RER, and the numerical density (Nv) of multivesicular bodies were determined in cells of the neck and base of glands. Circadian variation of the four variables was suggested in both experiments. Canalicular and microvesicular measurements suggested that a rhythm in gastric acid secretion may persist during fasting; a peak and trough, respectively, occurred in the late dark phase, as in our previous report on ad libitum-fed rats. Restriction of feeding to that which is normally the rat's resting phase caused an apparent 180 degree phase-shift in the rhythm. The data suggested, however, that additional factors may have influenced the cellular activity pattern. At all timepoints in both experiments cells of the neck of glands had higher RER and canalicular values than did cells of the base of glands. This suggests that parietal cells in glandular necks may be more active than those farther removed from the stomach lumen. There was no correlation between the Nv of multivesicular bodies and glandular location of the cells.