Leonard Ernest Bayliss was born on November 15, 1900, in London, England. The youngest son of Sir William Maddock Bayliss and Gertrude Starling Bayliss, sister of the physiologist Ernest H. Starling, Bayliss grew up near Hampstead Heath, on the northern fringe of London. His parents entertained generously and attracted many overseas physiologists to meet their British colleagues in an atmosphere of tennis, good fellowship, and unending, searching arguments about the fundamentals of physical, biological, and medical sciences. Here his father had his private laboratory and workshop although, like Starling, he was also a professor in the physiological laboratory of University College, London. In such a home Bayliss developed his “do-it-yourself” attitude toward apparatus and his capacity for exacting and informed analysis of scientific problems.