Chronic ingestion of non-steroidal anti-inflammatory medication is reported to delay or, in part, reverse development of polyps in the colon, but the mechanism for this effect is unknown. Using mRNA and immunoglobulin probes, specific for prostanoid receptors and for prostaglandin endoperoxide synthase (COX 1 and 2), we sought to define, by in situ and in vitro techniques, changes in PGE2 receptors and synthesis in cell populations of precancerous familial adenomatous polyposis (FAP) colonic mucosa. In FAP, expression of prostanoid receptors EP3 and EP4 among colonic lamina propria mononuclear and lateral crypt epithelial cells was robust, with 53.9+/-5.3% of mononuclear cells staining EP4+. When sections of normal colonic mucosa were examined by similar techniques, prostanoid receptor EP4 was expressed on only 21.3+/-1.2% of lamina propria mononuclear cells (including CD4+ T lymphocytes), as well as on surface and lateral crypt epithelium, and this distribution was found at the mRNA level as well. When receptor expression was quantitated by densitometry, immunoreactive EP3 protein on deep basolateral (but not other) FAP crypt epithelium was enhanced 2.8-fold over normal, and the number of prostanoid receptor EP4+ mononuclear cells by 2.5-fold. On the other hand, while COX 1 expression in mononuclear cells was prominent in normal and FAP mucosa, densitometric analysis showed immunoreactive prostaglandin endoperoxide synthase levels were further increased in FAP, due to a greater than fourfold elevation of COX 2 expression among mononuclear cells and epithelia. Our data suggest enhanced cell-specific prostanoid receptor expression and increased prostanoid synthesis in precancerous FAP mucosa.