The proliferative activity of newborn and adult mouse adrenal medullary cells was determined with light and electron microscopic autoradiography. The H3 thymidine labelling index of 2 weeks old mice adrenal medullary cells was about 9.4% and declined to less than 1% in adult mice. In electron microscopic autoradiography labelled norepinephrine as well as epinephrine cells could be seen. Only in 1 and 2 weeks old mice some morphologically undifferentiated cells were visible. In formaldehyde induced fluorescence combined with light microscopic autoradiography the fluorescence intensities of labelled and unlabelled medullary cells were measured. On average the fluorescence intensity of labelled cells was lower than that of unlabelled cells. The differences could be explained by a higher number of autoradiographic silver grains laying on the cytoplasm of labelled cells. These results give evidence that fully differentiated adrenal medullary cells are capable of division.