Abstract It was shown that better estimations were attained as compared with a direct microscopic method. Application of drawing ink at the gingival margin as one of the reference points to clarify the border between the tooth surface and the gingival epithelium was effective. Acceleration of the eruption rate at the beginning of a period of induced unimpeded eruption was not prominent in the first 24 hr. Diurnal variations of rate was inconspicuous. It was not possible to detect the effect of repeated frequent anaesthesia and handling of the rats. Average eruption rates per 2 hr ranged from 24 to 108 μm in both impeded and unimpeded mandibular incisors. Most were significantly different from zero. Experimental errors due to photographic procedures were relatively small. The standard deviation of the differences of the two observers' readings was about 50 μm with the photographic method which showed agreement with the comparable values obtained by a radiographic method by other investigators.