1.In order to characterize prostanoid receptors in the non-pregnant porcine uterus, effects of naturally occurring PGs(PGD_2, PGE_2, PGE_1, PGF_<2α> PGI_2) and synthetic prostanoid receptor agonists (cloprostenol, BW-245C, iloprost) on contractility of longitudinal(LM) and circular muscles(CM) were examined in vitro. 2.PGD_2, PGE_2, PGE_1, PGF_<2α> and PGI_2 increased muscle contractility of the LM. The ranking order of potency was estimated to be PGF_<2α>&gnE;PGE_2=PGE_1>PGD_2>PGI_2. On the contrary, PGE_1 and PGE_2 inhibited the muscle contractility at a high concentration (1-10μM). 3.PGD_2 inhibited the spontaneous contractions of the CM but PGE_2, PGE_1, PGF_<2α> and PGI_2 were less effective in causing the inhibition. The PGD_2-induced inhibition was potentiated by Ro20-1724 but not by zaprinast. PGD_2 treatment (for 5 min) significantly increased the tissue cyclic AMP level. 4.Cloprostenol (1 nM-10 μM) increased the contractility of both muscle layers but the LM was more sensitive to cloprostenol than the CM. 5.BW-245C (1 nM-10 μM) inhibited the spontaneous contractions of both muscle layers but the inhibition in the CM was more conspicuous than that in the LM. 6.Iloprost (10 nM-10 μM) increased the contractility of both muscle layers and these responses were not antagonized by SC19220 (5 μM). 7.Indomethacin (100 nM for 60 min) significantly decreased the frequency of spontaneous contractions in the LM. But spontaneous contractions in the CM were quitely insensitive to indomethacin. 8.The present results suggest that contractile (FP, EP_3) and relaxatory(DP, EP_2/EP_4) prostanoid receptors are present in the porcine myometrium in a muscle layer-dependent manner(FP, LM>CM ; DP, CM>LM). Endogeneous PGs involve in the initiation of spontaneous contractions in the LM.