The invention concerns pharmaceutically useful compounds of the formula I, in which R.sup.1, R.sup.2, R.sup.3, n, m and Het have any of the meanings defined herein, and their pharmaceutically-acceptable salts, and pharmaceutical compositions containing them. The novel compounds possess endothelin receptor antagonist activity and are useful, for example, in the treatment of diseases or medical conditions in which elevated or abnormal levels of endothelin play a significant causative role. The invention further concerns processess for the manufacture of the novel compounds and the use of the compounds in medical treatment.