Use of intermittent white noise (60 and 70 dB) as an additional synchronizer in the light period (12 h) gave a rise in the level of mitotic index and moved the maximal values 1 to 3 h forward in time. The effects increased with the sound level. Unskilled handling of the animals in itself could produce a significant decrease in mitotic index. Independent of the various external conditions injection of ‘HTdR (1 /μCi/g body weight; 6 Ci/m mol) increased the mitotic index during the first hours after injection.