Morphometrical analysis of the endocrine pancreas of senile 30-month-old rats revealed that the volume density as well as the numeric density of islets of Langerhans were much lower than in 24-month-old rats, which coincided with a much higher percentage of pycnotic nuclei in islet cells. The proportion and localization of the different categories of endocrine cells (A, B, D and PP) remained however unchanged with aging. The apparent problem of cell renewal observed in vivo in the very old age was detected earlier in vitro by tritiated thymidine incorporation. Such experiments showed that 24-month-old islet cells had a decreased labelling index when compared to 3-month-old cells. The proliferation capacity of the old cells could be partially increased by changing the serum concentration or type. Similarly as being more sensitive to serum factors, these cells underwent also more pronounced negative influence of high oxygen pressure on replication. A stereological analysis of the ultrastructure of non-degenerated B-cell nuclei revealed that with age, the relative volume of the condensed chromatin increased progressively at the expense of the dispersed form. This suggests that the still functioning senile B-cells could reduce their transcriptional activity.