The ability of Tetrahymena pyriformis to undergo synchronous division following release of inhibition with vinblastine was examined. The degree of synchrony was shown to be correlated with the period of time spent under the influence of vinblastine. Cells were inhibited for different periods of time with vinblastine and then washed free of the inhibitor with fresh medium. The increase in cell number and division index was followed subsequent to release of inhibition. Inhibition for a period of time equal to about two generation times was required to produce a complete doubling of the population during the first division. Inhibition for longer periods of time resulted in the population's increasing by more than a factor of two during the first division burst. The nuclear cytology indicates that the micronucleus is probably blocked in mitosis.

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