During World War II, she conducted experiments from a home laboratory, studying the growth of nerve fibers in chicken embryos which laid the groundwork for much of her later research. Her first genetics laboratory was in her bedroom at her home. In 1943, her family fled south to Florence, and she set up a laboratory there also. Her family returned to Turin in 1945.
In September of 1946, Levi-Montalcini accepted an invitation to Washington University in St. Louis, under the supervision of Professor Viktor Hamburger. Although the initial invitation was for one semester, she stayed for thirty years. It was there that she did her most important work: isolating the nerve growth factor (NGF) from observations of certain cancerous tissues that cause extremely rapid growth of nerve cells in 1952. She was made a Full Professor in 1958, and in 1962, established a research unit in Rome, dividing the rest of her time between there and St. Louis.
From 1961 to 1969 she directed the Research Center of Neurobiology of the CNR (Rome), and from 1969 to 1978 the Laboratory of Cellular Biology.
 Senator for Life
On August 1, 2001 she was appointed as Senator for Life by the President of the Italian Republic, Carlo Azeglio Ciampi. On 28–29 April 2006, Levi-Montalcini, aged 97, attended the opening assembly of the newly-elected Senate, at which the President of the Senate was elected; she declared her preference for the centre-left candidate Franco Marini. Levi-Montalcini, who is the senior member of the Upper House, chose not to be the temporary president on this occasion. She actively takes part in the Upper House discussions, unless busy in academic activities around the world. Due to her support of the government of Romano Prodi, she was often criticized by some right-wing senators, who accused her of "saving" the government when the government's exiguous majority in the Senate was at risk. She has been frequently insulted in public, and on blogs, since 2006, by both center-right senators such as Francesco Storace, and far-right bloggers for her age and Jewish origins.
Levi-Montalcini is currently the oldest living and the longest-lived Nobel laureate who, though hard of hearing and nearly blind, recently vowed to remain a political force in her country.