Prostaglandin I.sub.2 analogues of the general formula ##STR1## [wherein R.sup.1 represents a hydrogen atom, an alkyl group containing from 1 to 12 carbon atoms, an aralkyl group containing from 7 to 13 carbon atoms, a cycloalkyl group containing from 4 to 7 carbon atoms unsubstituted or substituted by at least one alkyl group containing from 1 to 4 carbon atoms, a phenyl group unsubstituted or substituted by at least one halogen atom, trifluoromethyl or phenyl group, or by at least one alkyl, alkoxy or alkylthio group containing from 1 to 4 carbon atoms, or represents a group --C.sub.m H.sub.2m COOR.sup.6, --C.sub.n H.sub.2n OR.sup.7 or ##STR2## in which m represents an integer of 1 to 12, n represents an integer of 2 to 12, R.sup.6, R.sup.8 and R.sup.9, which may be the same or different, each represent an alkyl group containing from 1 to 4 carbon atoms, and R.sup.7 represents a hydrogen atom or an alkyl group containing from 1 to 4 carbon atoms, R.sup.2 represents a hydrogen atom or a methyl or ethyl group, R.sup.3 represents an alkyl group containing from 1 to 4 carbon atoms, R.sup.4 represents a hydrogen atom, or an alkyl group containing from 1 to 4 carbon atoms, R.sup.5 represents an alkyl group containing from 1 to 10 carbon atoms, Y represents ethylene or trans-vinylene, the hydroxy group attached to the C-11 carbon atom is in .alpha.-configuration, and the wavy line attached to the C-15 carbon atom is in .alpha.- or .beta.-configuration or a mixture thereof] and non-toxic acid addition salts thereof and, when R.sup.1 represents a hydrogen atom, non-toxic salts thereof are new compounds possessing pharmacological properties typical of the prostaglandins.