Following graduation in 1872, he participated in the founding of the Argentine Scientific Society. He embarked on the first of the series of scientific expeditions that made him well known: a survey of Río Negro Territory, largely uncharted country. In January 1876, he reached Lake Nahuel-Huapi in the southern Andes, and on February 15, 1877, he discovered and named Lake Argentino. He also explored numerous rivers in Patagonia. On March 2, he discovered and named Mount Fitz Roy, after the commander of the expedition of in the 1830s. The native people also called it "Chalten".