Postnatal uterine development is marked by periods of tissue remodeling. The objective of the present study was to examine the role of tissue inhibitor of metalloproteinase-1 (TIMP-1), a regulator of tissue remodeling events, during postnatal uterine development and to assess the phenotypic consequences of disruption of the TIMP-1 gene product during this time period. To accomplish this goal, wild-type and TIMP-1 null mice were sacrificed at Postnatal Days (PNDs) 5, 10, 15, 20, and 25 and uterine morphology, TIMP expression and matrix metalloproteinase (MMP) activity were assessed. In wild-type mice, TIMP-1 mRNA steady-state levels were highest at PND 5, after which expression decreased. TIMP-2 and TIMP-3 expression in wild-type mice showed no significant changes from PND 5 to 25. In TIMP-1 null mice, TIMP-2 and TIMP-3 expression patterns were similar to those in wild-type counterparts with the exception that, at PND 10, TIMP-2 and TIMP-3 expression was significantly lower in the null mice. Endometrial gland number and uterine histology were similar between genotypes at PNDs 5 and 10, but at PNDs 15 and 20, endometrial glands were more abundant in TIMP-1 null mice. Associated with the increased gland density in the null mice was an increase in total MMP activity above the levels expressed in wild-type mice. In summary, disruption of the TIMP-1 gene product is associated with reduced TIMP-2 and TIMP-3 steady-state mRNA levels, elevated MMP activity, and accelerated endometrial gland formation. We conclude that, during early postnatal uterine development, TIMP-1 may be critical for proper endometrial gland development.