Barlow's first major success came in 1966 when she made her debut at the Komische Oper Berlin as Donna Anna in Wolfgang Amadeus Mozart's Don Giovanni.
In 1967 Barlow sang her first Isolde in Wagner's Tristan und Isolde in Kiel, a performance which didn't put her on the map but which did lead to an engagement to sing the part at the 1968 Festival dei Due Mondi in Spoleto, Italy opposite Claude Heater as Tristan.