We have used the highly sensitive in situ terminal deoxynucleotidyl transferase method, applied to ultrathin sections, to investigate the location of DNA within nucleoli of various animal cells. In all the nucleoli studied, intense labelling is revealed over the peri- and intranucleolar condensed chromatin. Gold particles are also consistently found over the fibrillar centres, especially at their periphery, namely in the border area between the fibrillar centres and the dense fibrillar component, whereas the dense fibrillar component itself seems to be free of label in nucleoli in which these two compartments can be distinguished. We conclude that, in transcriptionally active nucleoli of this type, DNA is a characteristic constituent of the fibrillar centres, distinguishing them functionally from the dense fibrillar component. Some nucleoli exhibit neither fibrillar centres nor a dense fibrillar component, but have a single, albeit heterogeneous accumulation of fibrillar material; gold particles are consistently seen over some parts of this fibrillar compartment. This suggests that certain parts of the fibrillar material are functionally similar to the fibrillar centres of those nucleoli that possess them.