In the seedlings of Sinapis alba, the lag-phase between the application of 5-FUDR and the beginning of the inhibition of elongation growth and the inhibition of DNA-synthesis has been studied. The elongation was retarded after 7 h, and then, depending on the concentration of the FUDR, was completely stopped. In the cotyledons the DNA-synthesis was strongly reduced after about 50 minutes, and in the hypocotyls a lag-phase of less than 30 minutes was observed. With the addition of thymidine the DNA-synthesis was immediately resumed, while the growth began with a lag-phase of 5–7 h. In every case the change in the DNA-synthesis preceded the change in the elongation growth. The inhibition of elongation growth could, therefore, be the consequence of inhibition of DNA-synthesis.