Some studies have shown that pinealectomized female rats present a large concentration of collagen fibers in the endometrium. The lack of light results in morphologic alterations in the uterine lamina propria and in the muscular layer, reducing thus the uterine weight. Therefore, this work aimed to analyze ultra-structurally the influence of melatonin in the remodeling of the collagen in the endometrium, in the estrus phase, in pinealectomized adult rats or submitted to lack of light. The experimental groups: I- sham-pinealectomized maintained in a clear/dark cycle 12/12 hours, during three months; II- pinealectomized rats maintained in a clear/dark cycle 12/12 hours, for three months; III- normal rats maintained in clear/dark cycle 12/12 hours for three months; IV- normal rats maintained in lack of light, for three months. Fragments of a third of uterus average were gathered, fixed in Karnowsky solution and processed for Transmission Electron Microscope analysis. The results showed that the animals of the group II presented greater incidence of the estrus phase, in a percentile of 60%, and reduction of the proestrus phase. In the groups I, III and IV it had an equivalence of the estrus phase, however the animals of group IV showed a greater incidence of the diestrus phase with an average percentile of 40%. The ultra-structural analysis of the lamina propria of the endometrium in the animals of the groups I and III revealed several fibroblasts with nucleus presenting a little dense chromatin, and cytoplasm contents a great quantity of rugous endoplasmic reticulum. In the animals of the group II was observed a great quantity of fibroblasts, with several developed rugous endoplasmic reticulum, in spite of the great quantity of collagen fibers. In the animals of group IV themselves, few fibroblasts with small quantity of cytoplam that presents a scarce profile of rugous endoplasmic reticulum. Therefore, one can conclude that melatonin may perform a regulator factor in the collagen fibers synthesis by fibroblasts