The purpose of the present study was to determine whether a force of 20 cN can be biologically active for tooth movement and to examine the pain intensity during the application of light (20 cN) or heavy (200 cN) continuous forces for 7 days. In the first experiment, a force of 20 cN was applied to eight canines in five volunteers. The mean tooth movement during 10 weeks was 2.4 mm. In the second experiment, two forces of 20 or 200 cN were applied to maxillary premolars in 12 male subjects (aged 24-31 years) to measure pain intensity for 7 days. Spontaneous and biting pain were recorded every 2-4 hours on a 100 mm visual analogue scale (VAS). Wilcoxon signed-rank test was used for statistical analysis. Comparing the VAS score at force initiation with the other time points, there was no significant difference in spontaneous pain for either group, or in biting pain for the light-force group. However, biting pain in the heavy-force group during the time period from 6 to 156 hours was significantly (P < 0.05) greater than that at force initiation. Comparing the VAS scores between the light- and heavy-force group, VAS scores for biting pain in the heavy-force group during the time period from 8 to 100 hours was significantly (P < 0.05) greater than that in the light-force group. A force of 20 cN can move teeth, but pain intensity while biting may be greater approximately 8 hours to 5 days following the application of heavy continuous force compared with light force.