A comparison of tooth movement cycles and changes in alveolar tissue mineral densities was made between young (21 to 28 days old) and adult (90 to 100 days old) rats. An initial 60-g mesial tipping force was applied to the maxillary first molars; tooth movement was estimated by measuring the opening between first and second molars, and tissue mineral density by sample ash weight per cubic centimeter. A characteristic three-part tooth movement cycle was found in both groups of rats. This cycle consists of an early "instantaneous" movement that is a function of the viscoelastic properties of the tissues, a delay period during which little tooth movement occurs due to hyalinization and undermining resorption, and a late period during which bone remodeling and tooth movement occur. Compared with the adult group, the amount of "instantaneous" movement in the young rats was greater (P less than 0.001), the delay period was shorter, and the rate of late tooth movement was faster (P less than 0.001). The young rats had significantly lower (P less than 0.05) mineral densities before orthodontic treatment. Both groups experienced a slight but insignificant increase in density followed by rapid declines to a nadir that was not different between the groups. The young rats reached this point by day 5 and the adults by day 7. The point of lowest mineral density occurred in the treated alveolar tissues toward the end of the delay period for each group and was followed by a return to control values at rates that were not significantly different between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)