Prostaglandin I.sub.2 analogues of the formula: ##STR1## (wherein Y represents trans-vinylene or ethylene, R.sup.1 represents hydrogen or an alkyl group containing from 1 to 12 carbon atoms, R.sup.2 represents hydrogen, methyl or ethyl, R.sup.3 represents a single bond, or an alkylene group containing from 1 to 4 carbon atoms, R.sup.4, R.sup.5 and R.sup.6, which may be the same or different, each represent hydrogen or an alkyl group containing from 1 to 8 carbon atoms, n represents 3, 4, 5 or 6, the wavy line attached to the carbon atoms in positions 11 and 15 depicted in the formula represents .alpha.- or .beta.-configuration or mixtures thereof, and the double bond between C.sub.6 -C.sub.6 is Z or E) are new compounds possessing pharmacological properties typical of prostaglandins, for example PGI.sub.2.