Adrenaline (A)- and noradrenaline (N)-cells in juxtacortical and central areas of adrenal medullas of nonoperated, sham-operated, and pinealectomized male rats (n = 125) were investigated by quantitative electron and light microscopy. Animals were killed at eight time points during a standardized 24-h, light-dark (12:12) cycle 14 d after surgery. Diameters of nucleoli (n = 9,600) and the nucleolar margination rate were studied chiefly by light microscopy. Major findings include the following: Twenty-four-hour changes occurred in nucleolar diameters in adrenomedullary chromaffin cells in nonoperated animals, with the peak in the late dark phase (X +/- SD, A-cells: 1.29 +/- 0.06 micron; N-cells; 1.23 +/- 0.05 micron) and the minimum in the mid-light (A-cells: 1.16 +/- 0.04 micron) or early dark (N-cells: 1.14 +/- 0.03 micron) phase. These changes were markedly dampened (diminished) in sham-operated animals. Since this dampening of time-of-day changes after sham-operations was not seen in A-cells after pinealectomy, mediation by the gland of this dampening is suggested. Pinealectomized animals showed 24-h changes in nucleolar diameters, but these changes differed from those of nonoperated controls in their temporal and phase relations. For example, there was phase advance in A-cells in the juxtacortical area and desynchronization in N-cells. Furthermore, rhythm amplitude was slightly increased in A-cells and slightly decreased in N-cells after pinealectomy. A- and N-cells differed from each other consistently and significantly in the quantitative morphology of their nuleoli and nuclei, and slightly in the patterns of their time-of-day changes in nucleolar size. Nucleolar size usually showed a tendency for a regional gradient, decreasing slightly from periphery toward the center. These observations contribute to our understanding of the chronobiology and functional relations of adrenomedullary chromaffin cells, and of the effects of pinealectomy and sham operations upon this system.