Recently, uterine movement-induced mechanical stress was noticed to have possible effects on endometrial functions. In this study, we addressed the possible effect of mechanical stress on the process of decidualization of endometrial stromal cells (ESC). ESC were cultured on flexiblebottomed culture plates. After decidualization was achieved with estradiol and progesterone for 12 days, cultures were continued for 24 h with or without cyclic stretch (25% elongation) in serum-free conditions at a rate of 2 cycles/min using a computer-operated cell tension system. Concentrations of insulin-like growth factor-binding protein-1 (IGFBP-1), a marker of decidualization, in the conditioned medium were measured by specific ELISA, and IGFBP-1 mRNA expression in the ESC was measured by RT-PCR. Cyclic stretch remarkably increased IGFBP-1 secretion from decidualized ESC. It also increased IGFBP-1 mRNA in decidualized ESC. The increase in IGFBP-1 secretion was inhibited by actinomycin D but not by indomethacin, PD-98059, or H-89. Conditioned medium of decidualized ESC cultured with cyclic stretch increased IGFBP-1 secretion from decidualized ESC cultured under stationary conditions. These findings imply that uterine movement modulates decidualization of the endometrium and has a regulatory effect on reproduction.insulin-like growth factor-binding protein-1; endometrium; decidualization; prostaglandin E2; indometacin DECIDUALIZATION, a process of endometrial differentiation, is essential for embryo implantation and maintenance of pregnancy. During the process of decidualization, characteristics of endometrial cells change profoundly. This was demonstrated by marked alterations in the expression of numerous genes in decidualized human endometrial stromal cells (ESC) in vitro (17). Progesterone is a well-known inducer of decidualization, whereas prostaglandin E 2 (PGE 2 ) and relaxin enhance the process via activation of cAMP signaling (8,11,16,20). However, the complex mechanisms that regulates decidualization remains to be elucidated.Recently, uterine movement, a mechanical process, has been noticed to play important roles in fertility (4). Subendometrial myometrium exerts wave-like activity throughout the menstrual cycle (3, 13), and the movement is believed to support embryo and sperm transfer. The peristaltic uterine movement is also suggested to be involved in other uterus-related events such as endometriosis and menstruation. Recently, we (9) suggested that the subendometrial myometrial movement is transduced to a biochemical signal in the endometrium and may play roles in endometrium-related physiological and pathological events.Given that the mechanical stress associated with uterine peristalsis alters biochemical characteristics of the endometrium, it is plausible that decidualization, a differentiation process of the endometrium, may be affected by the uterine movement. To address the issue, we conducted an in vitro study to see the effect of mechanical stretch on the production of IGFBP-1, a representative marker of d...