Summary Exposure of quiescent cultures of bovine granulosa cells to epidermal growth factor (EGF) stimulates the cells to enter S-phase. Addition of the calmodulin inhibitor, trifluoperazine (TFP), reduces the rate at which these cells enter S-phase. Half-maximal inhibition occurs with a concentration of 3 × 10−6M TFP. The inhibitory effect of TFP only occurs if the drug is added to the cell cultures during the first 5–6 hours of G1, but not if it is added during the second half of G1. Similarly, addition of TFP during S-phase does not affect the number of cells entering S-phase, however, it does reduce the rate of incorporation of 3H-thymidine into acid-precipitable material. TFP does not inhibit the binding of EGF to these cells.