The refrain from “Old MacDonald Had a Farm” is “E-I-E-I-O.”
The refrain returns after the farm’s name and after each animal verse, such as “And on that farm he had a cow.” The song then adds the animal’s sound, making it cumulative as more animals and noises are introduced.
The song has a complicated history. A possible ancestor is “In the Fields in Frost and Snow,” associated with Thomas d’Urfey’s 1706 opera The Kingdom of the Birds. That version has a similar farm setting and repeated animal sounds, but it does not establish that d’Urfey wrote the modern song.
Traditional British and American versions circulated for generations before the familiar form was standardized. A 1917 variant called the farmer “Old Macdougal,” while early recordings using the now-standard name and tune appeared in the 1920s. The refrain is sometimes written with hyphens, but the answer is the same letter sequence.