. A study of the effects of prostaglandin E₂ (PGE₂) and eleven synthetic analogues on the guinea‐pig isolated ileum preparation has revealed three distinct contractile actions, each associated with a different prostaglandin E (EP‐) receptor subtype. In addition, PGI₂ (prostacyclin) and its stable analogues can activate prostaglandin I (IP‐) receptors to elicit both contraction and relaxation of the ileum.

. Two of the PGE actions involve direct stimulation of the smooth muscle, being unaffected by 1 μm morphine treatment. One action is blocked by AH 6809 at micromolar concentrations and ICI 80205 and 16,16‐dimethyl PGE₂ are particularly potent agonists. Activation of EP₁‐receptors appears to be involved. The second action is unaffected by AH 6809; sulprostone and MB 28767 are potent agonists. Comparison with agonist potency rankings on the guinea‐pig vas deferens indicates that EP₃‐receptors may be involved.

. The third PGE effect and the stimulant PGI effect are blocked by morphine, indicating enteric neurones and/or sensory nerve terminals as sites of action. EP₂‐receptors may be involved in the PGE action, in view of the marked effect of morphine on the contractile actions of misoprostol, 11‐deoxy PGE₂‐1‐alcohol, 11‐deoxy PGE₁ and butaprost, all of which show some selectivity for EP₂‐receptors. The PGI action is most easily studied with cicaprost (EC₂₅ = 1.3 nm), since iloprost, carbacyclin and to a lesser extent PGI₂ also have agonist activity at EP₁‐receptors.

. The contractile action of 17‐phenyl‐ω‐trinor PGE₂ on the ileum is unaffected by morphine. Since this analogue shows only weak agonist activity on the rabbit jugular vein (EP₂ preparation) and guinea‐pig vas deferens (EP₃), it may be a more useful standard agonist than PGE₂ in EP₁‐receptor studies.

. In the presence of morphine and AH 6809, cicaprost inhibits histamine‐induced contractions (IC₂₅ = 22 nm). PGI₂ and iloprost show mixed inhibitory/potentiating actions, whereas carbacyclin only potentiates histamine contractions. This IP‐receptor‐mediated inhibition may account for the bell‐shaped log concentration‐response curve of cicaprost (no inhibitors present) and the very marked block of iloprost‐induced contractions by AH 6809.

. We have found no evidence for either IP‐receptors mediating direct contraction or EP‐receptors mediating inhibition of the ileum longitudinal smooth muscle, as has been suggested in the literature.

. In view of the complexity of prostanoid action on the guinea‐pig ileum we feel that the preparation must be used with caution to ascertain the EP₁ agonist or antagonist potencies of novel compounds.