BACKGROUND. Colon muscle strips and cells from female patients with slow transit constipation (STC) exhibit impaired motility, signal transduction abnormalities characterized by down regulation of Gq/11 and up regulation of Gs proteins, decreased COX-1 and TxB2 levels, increased COX-2 and PGE2 levels and over expression of progesterone receptors (PR). Progesterone (P4) treatment of normal cells reproduced those motility and signal transduction abnormalities. AIMS: to examine whether over expression of PR-B reproduces these abnormalities by rendering the cells more sensitive to physiological concentrations P4. METHODS: Cultured human colon muscle was transfected with a plasmid DNA expressing PR-B. The mRNAs of PR, COX-1, COX-2 and Gq/11 were determined by qPCR. Their protein expression was determined by Western blot and prostaglandins were measured by radio- immunoassay. RESULTS: Cultured muscle cells maintain their phenotypic features determined by myosin light chain and h-caldesmon antibodies. Control and transfected muscle cells responded to P4 10^-6 M. In contrast, transfected muscle cells with PR-B responded to lower P4 concentrations of 10^-7 M. This P4 concentration reduced MLC phosphorylation induced by CCK-8 (10^-8 M), downregulated Gq/11 and decreased COX-1 and TxB2 levels. It upregulated Gs proteins. It also increased COX-2 and PGE2 levels. CONCLUSION: overexpression of PRB renders the cells more sensitive to physiological concentrations of P4. These results are consistent with the hypothesis that overexpression of PR-B contributes to the motility and signal transduction abnormalities observed in female patients with STC with normal serum levels of P4.