The present study was undertaken to investigate the structural changes in both cholinesterase(ChE)-positive nerve fibers and adrenergic nerves with formaldehyde-induced fluorescence in pregnant and postpartum uteri of both the albino rat and guinea pig. Particular attention was directed to the relationship between these changes and the local factors associated with the growing fetus. ChE reaction was absent in the control and pregnant uterus of the guinea pig. In the albino rat, there were signs of degeneration in pregnancy. These were evidenced by vacuolation of large nerve trunks and the presence of focal segments with very faint reaction along the course of the nerve bundles. Myometrial segments from fetus-containing horns showed some fragmented nerve fibers, but at the same time some other normal ones. Most of the fine nerve bundles gave a weak reaction. Three weeks after delivery, multiple ChE fibers were found in the uterus of the albino rat. The normal appearance was, however, not regained and some nerve fibers were still fragmented. Noradrenergic (NA) nerve fibers were disintegrated and markedly reduced in number in the myometrium of the pregnant uterus of both the guinea pig and albino rat, particularly in the uterine horns that were distended by fetuses. The number of NA fibers was not significantly reduced in the tubal ends of the albino rat uterus. Three weeks after delivery, normal NA fibers were seen in the myometrium of both the albino rat and guinea pig uterus. Nerves with reduced fluorescence reaction were observed less frequently.