Caffeine has several highly visible effects on mitosis in eggs of the sea urchin Strongylocentrotus purpuratus. It increases the cell cycle length in a dose-dependent manner and at a concentration between 5 and 10 mM it prevents the normal separation and function of the mitotic centers, resulting in periodic monaster formation rather than division. If fertilized eggs are placed in seawater containing 10 mM caffeine at prometaphase, the spindle rapidly shrinks, moving the mitotic centers to the middle of the metaphase plate in less than 10 min. Within 30 min the eggs recover their ability to polymerize microtubules to form a large monaster, and from this time on they continue to cycle monasters in the presence of caffeine, with a period of about 150 min, where the normal second division time is 55 min. Other cell cycle functions, such as chromosome condensation, nuclear membrane breakdown and reformation, attempted furrowing and surface membrane addition continue for at least several cycles, but at the slower rate. Although the evidence strongly suggests that the rapid breakdown of the prometaphase spindle results from a calcium release induced by caffeine, these experiments do not rule out other physiological effects of caffeine on the longer-term changes in cycle length, centriole function, and ultimately on cell survival.