We analyzed IGFBP-1 gene promoter activity by transient transfection during the progressive decidualization of human endometrial stromal cells. A time study over a 13-day culture period showed that the promoter activity increased exponentially to> 10 (4) fold in cells treated with MPA and RLX correlating with the secretion rate and steady-state mRNA levels of the endogenous gene. Deletion analysis showed that two regions in the IGFBP-1 gene promoter are responsible for the activation of the IGFBP-1 gene. The basal promoter region between-1 and-300 bp contains multiple sections of functional elements homologous either to CRE, PRE, or CCAAT. The major difference of IGFBP-1 gene activation in endometrium and the hepatic system lies in the distal promoter region, between-2.6 and-3.4 kb, which mediates 95% of the total promoter activity derived from-3.3 kb to+ 68 bp. Functional and binding analysis in the distal promoter region showed that multiple Sp1 elements interacting with a novel Sp3 transcription factor activates the hIGFBP-1 gene promoter.