Earlier work has shown that there is a periodic change in the rate of production of CO2 during the cell cycle of fission yeast and that this periodicity persists after a block to the DNA-division cycle and also after a block to protein synthesis. It appears that there is a periodic control or 'oscillator' affecting CO2 production that is normally closely entrained to the cell cycle, but which can 'free-run' after a block. In this paper, we examine what events in the DNA-division cycle can generate the entrainment signals and what is the nature of such signals. In the first set of experiments, CO2 production was measured by manometry during induction synchrony produced by blocking the DNA-division cycle in an asynchronous culture for a period and then releasing the block. Synchronous cell division occurs after the release with cell cycles shorter than normal. After release from a block imposed by shifting up the mutant cdc2.33 to the restrictive temperature, oscillations in CO2 production started rapidly and remained closely entrained to the division cycles (with slightly different patterns and timing from those after selection synchrony). This showed that there was an entrainment signal but did not show whether it came from start, the S period or mitosis. A similar experiment with cdc10.129 showed that an early signal came from either start or the S period, as did an experiment with release from N-starvation. The results with cdc25.22 were similar to those with cdc2.33. After a block with hydroxyurea, there was entrainment but with no signs of the early signal that occurred with cdc10. This showed that the early signal came from start and not from the S period. In a second set of double-block experiments, the first block was followed by a second different block. With cdc25.22 followed by MBC (an inhibitor of nuclear division) the cells passed through a narrow window of the cell cycle between the transition point of cdc25.22 and mitosis. This was sufficient to start the oscillations, showing that an entrainment signal could be generated at about the time of mitosis. The results from using hydroxyurea followed by cdc2.33 showed no genuine oscillations, confirming the conclusion from the single hydroxyurea block. The results from using hydroxyurea followed by cdc10.129 confirmed the existence of a mitotic signal.(ABSTRACT TRUNCATED AT 400 WORDS)