Progression of the HeLa cell through its life cycle is accompanied by centriolar replication and pericentriolar changes that are in synchrony with DNA synthesis and mitosis. The first signs of preparation for replication occur during G(1) at which time the two orthogonal centrioles separate. Replication by budding begins at/or near the initiation of DNA synthesis and is completed by G(2). Pericentriolar changes which probably are causally related to spindle tubule formation occur at this time and include the appearance of vesicles, electron-opaque bodies, and an amorphous pericentriolar halo. These phenomena begin to disappear by late prophase, and the remainder of mitosis manifests decreasing centriolar and pericentriolar activity.