The frequency of mitoses of adrenaline (A) cells and noradrenaline (N) cells in the adrenal medulla of nonoperated (NO), sham‐operated (SPX), and pinealectomized (PX) male, 53‐day‐old Holtzman rats (n = 133) was investigated by means of light microscopy. Animals were killed at eight time points during a standardized 24‐h light‐dark (12:12) cycle 14 days after surgery. Mitotic indices (n/1,000) were determined in sections of adrenal medulla fixed with glutaraldehyde and OsO4. Overall frequency of mitoses was extremely low (mitotic index: 0.73 = 115/157,223). Daily mean mitotic index was maximum in A cells (0.83) and minimum in N cells (0.52) of PX group but did not show statistically significant differences between cell types or experimental groups. Neither cell type in NO animals showed 24‐h changes in mitotic index, but cells in SPX animals did, with highest value in the late dark phase and lowest in the late light phase, when values of two cell types were combined (P<0.01–0.001). In PX animals, mitotic index followed a similar but more distinct 24‐h change in A cells (P<0.009), but not in N cells, resulting in different time‐of‐day changes between two types of cells (P<0.01–0.05). The mitotic index was higher in PX than in control (NO and SPX) animals in the middark phase (P<0.05) and lower in operated (SPX and PX) than in nonoperated (NO) animals from late light to the early dark phase, suggesting that the latter was possibly due to a residual effect of the surgery. These results are consistent with the interpretation that the pineal has an inhibitory action on A cells and may coordinate the two types of cells in their mitotic activity, especially in the middark pha