British physiologist who was the first to trace the movements of sodium and potassium during the transmission of a nerve impulse by using radioactive sodium and potassium.

After receiving his Ph.D. in 1949 from the University of Cambridge, Keynes joined its faculty. He was head of the physiology department of the Agricultural Research Council's Institute of Animal Physiology at Babraham in 1960–64 and assumed the directorship of the institute from 1965 to 1973. From 1981 to 1984 he was president of the International Union for Pure and Applied Biophysics. In 1959 he became a fellow of the Royal Society of London.

Keynes contributed many papers to scientific journals, primarily on ion (i.e., charged atom) transport in tissue and on the biochemistry and thermodynamics of the electrical organs found in some eels and fishes.