Release of CHO-K1 cells from plateau or stationary phase and reentry into the cell cycle is specifically and reversibly blocked at two distinct sites by the anticalmodulin drug W13. The first block occurs early during release while the cells are still at G0/G1, whereas the second occurs later in reentry during early S phase. As determined by radioimmunoassay, calmodulin levels undergo changes at three distinct steps in plateau-phase entry and release. First, the entry of exponentially growing cells into plateau phase is accompanied by an increase in the calmodulin level. The second change is a reduction in the calmodulin content of cells within the first hour following release from plateau phase. The third change is the subsequent increase in calmodulin levels, which precedes entry of the cells into S phase. Analysis of calmodulin mRNA levels by dot-blot hybridization demonstrates that the changes in calmodulin protein are preceded by changes in calmodulin mRNA. Furthermore, whereas a decrease in CaM mRNA is observed within the first hour following plateau release, no such decrease is observed for beta-actin mRNA, suggesting that this decrease may be selective for calmodulin. This selectivity is further substantiated by the fact that identical changes in calmodulin and calmodulin mRNA are observed in cells released from plateau by two different techniques. Taken together, these data suggest that calmodulin may play an important role in the reentry of cells into the cell cycle.