Porcine iris sphincter muscle strips contracted in response to carbachol. The tissue contraction was inhibited by prostaglandin (PG) E2 but not by PGF2α. In order to investigate the effect of PGs on the iris cells, the porcine sphincter muscle cells were grown in culture to a confluence and characterized. Using the secondary culture of cells, the effect of PGs on carbachol-induced cell contraction was investigated. Both PGE2 and PGF2α at 100 μM blocked cell contraction completely. The concentration required to inhibit 50% of the maximum contraction in 15 minutes was 10⁻⁶ M for PGE2 and 10⁻⁶ - 10⁻⁷ M for PGF2α. Using PGE2 receptor subtype agonists (EP2 agonist, 11-deoxy-16, 16 dimethyl PGE2 and EP3 agonist, sulprostone), PGE2 receptor involved in the inhibition of carbachol-induced contraction was identified to be of the EP2 subtype. In support of this characterization, the addition of PGE2 to cultured porcine sphincter muscle cells increased intracellular cAMP level. The discrepancy in PGF2α effect on carbachol-induced sphincter muscle contraction between iris tissue strips and cultured cells suggests that nonmuscular cells may be involved in the modulation of the PGF2α effect on sphincter muscle cells in vivo.