1-beta-D-Arabinofuranosylcytosine (ara-C) affects the passage rate of cells from S to G2 phase to a greater extent than from G1 to S. This finding is consistent with the fact that the compound may have its major biological effect during the latter part of S and the early part of G2. The magnitude of these effects is related to the dose and duration of ara-C administration. Inhibition of DNA synthesis can occur without disturbances in transit time.