The present study has investigated the expression of the chemokines IL-8 and MCP-1, and COX-2 in the human non-pregnant and pregnant endometrium. Both IL-8 and MCP-1 were localised by immunohistochemistry to the perivascular cells of all blood vessel types in the endometrium. Similarly COX-2 immunostaining was present in the vasculature and glandular epithelium. Reduced immunoreactivity was detectable in the periovulatory, early and mid secretory phases. Maximal levels were identified in the premenstrual or late secretory phase, coinciding with progesterone withdrawal and in addition, with the accumulation of leukocytes in uterine stroma. In first trimester decidua, low levels of expression were observed in IL-8 and COX-2, with slightly higher levels for MCP-1. This pattern was confirmed by measurement of mRNA levels by non-competitive semi-quantitative RT-PCR. Equal loading of RNA samples was ensured by measurement of GAP-DH expression levels. RT-PCR product yield for GAP-DH, IL-8, MCP-1 and COX-2 was determined by ELISA. Maximal expression was observed in menstrual phase samples. Decidual mRNA concentrations were consistent with immunohistochemical data. The role of progesterone in regulation of local mediator expression was examined by immunohistochemical analysis of endometrium and decidua obtained from women with clinically perturbed progesterone concentrations. Additionally, RNA extracted from tissue from timed endometrial biopsies was subjected to RT-PCR for GAP-DH, IL-8, MCP-1 and COX-2 as before. 48 hours after exogenous progesterone withdrawal a significant elevation in IL-8 and COX-2 mRNA expression was detected. Further supportive evidence for an inhibitory role for progesterone was the low expression in women administered hCG to extend the lifespan of the corpus luteum.