The mesial gingiva of the maxillary first molar of SAM-R/1/Iw (senescence-accelerated mouse, resistant/1/Iwate) and the P/2/Iw (prone/2/Iwate) were studied morphologically by quantitative and qualitative methods as a function of age (one-, two-, six-, 12-, and 16-month-old). In this region, neither plaque nor periodontitis has been found (Sashima et al., 1990a). Statistical analyses of the gingival tissues in this region in both strains showed that, with age, the length of the junctional epithelium (JED) increased, the depth of the gingival sulcus (GSD) did not change, and the height of the gingival crest (GCD) decreased. Therefore, long-junctional epithelium developed, and gingival recession occurred, since a periodontal pocket did not develop with age in either strain. In particular, there was a strong correlation between the JED and age in both strains (r = 0.84, p less than 0.05 for the R/1/Iw, and r = 0.92, p less than 0.05 for the P/2/Iw). The JED, GSD, and GCD of the P/2/Iw were higher than each of those in the R/1/Iw after six months of age; however, there were no significant differences between the R/1/Iw and the P/2/Iw for any age group (one-, two-, six-, 12-, and 16-month-old).