The acclamation Kyrie eleison (Lord have mercy!) antedates Christian worship and is the one part of western liturgy that remained in Greek throughout the Middle Ages. It was used in various forms of pagan worship including the imperial cult in which the emperor was called kyrios (lord). Since Christians early on adopted the title lord to call on Jesus of Nazareth whom they believed to be God’s annoined (that is to say Christ or Messiah), the acclamation kyrie eleison was quickly incorporated into Christian worship. The fourth- century nun Egeria reported in her Travels (25: 5) that worshippers employed the acclamation during the lamplighting ceremony of vespers in Jerusalem.
Christians principally employed the acclamation as a popular response in litanies. A deacon or other liturgical leader would intone a short supplication, and the assembly would respond kyrie eleison. In Rome, litanies with kyrie eleison were sung after antiphonal psalms at the end of long processions on important holidays to the stational churches. The incorporation of the kyrie into the opening rites of the Mass appears to be in imitation of these large-scale processions. The solemn entrance into the Mass became a short version of the stational processions, one psalm (the introit) followed by a litany with the responsekyrie eleison. By the sixth century, Pope Gregory the Great tells us that the litanic form was dropped on weekdays and only the three-fold or nine-fold singing of kyrie eleison was left. Gregory also mentioned the custom of alternating the kyrie eleison with a new acclamation christe eleison.
Someone coming from Sicily has told me that some friend, whether Greek or Latin I do not know, but having great zeal for the Holy Roman Church, has grumbled about my changes, saying: ‘Why do you wish to amalgamate with the Church of Constantinople by following its customs in all things?’ But to him I say: ‘Which of its customs do we follow?’ He responds: ‘Because the Kyrie eleison is sung.’ But we have not nor do we sing the Kyrie eleison as the Greeks do, for among the Greeks all sing it at the same time, whereas among us the clergy sing it and the people respond. We all sing Christe eleison as many times [as Kyrie eleison], and the Greeks do not. Finally, in daily masses we omit the other things which are usually sung, and sing only Kyrie andChriste eleison, so that we might spend a little more time in this prayer of supplication. (Gregory the Great, Epistles 9: 26, trans. Baldovin, Urban Worship, 244-245)
Like the Gloria in excelsis the Kyrie eleison was sung to hundreds of different melodies across Europe. The first sources with notation contain both melodies that have only the acclamations Kyrie eleisonand Christe eleison as well as melodies that contain both Latin verses and the Greek acclamations. Until fairly recently many scholars argued that these Latin verses were tropes, that is to say text added later to an originally melismatic chant. But, especially given the origins of the chant as a litany, there is a good possibility that these chants witness to the practice of a full litanic form of the Kyrie eleison for Sundays and holy days and a textually simpler version for ordinary days described by Gregory in his letter.
Ockegham: Missa pro Defunctis (Kyrie)