Thirty rats were divided into 3 equal groups. Following 9 days of induced unimpeded eruption, a single partial pulp resection was performed on the repeatedly-shortened lower left incisor of one group. A total of 3 resections, each repeated after 48 h, was performed on the animals in a second group. The lower left incisor was shortened but the pulp was not resected in the 3rd sham-operated control group. Eruption rates of all of the incisors were recorded daily. Partial resection of the pulp increased the unimpeded rate of the experimental incisors in the first group by 50 per cent at 24 h after operation and an increase occurred 3 times in the second group of rats (by 45, 36 and 25 per cent respectively). The rate returned to control levels 24 h later. No significant differences were observed between the eruption rates of the right maxillary and mandibular incisors of the animals in the 3 groups. The increased rate of eruption may be induced by local regulators from the traumatized pulp that are transmitted to components of the periodontal ligament which provide the force for eruption.