John Scott Haldane CH FRS (; 2 May 1860 – 14/15 March 1936) was a Scottish physiologist famous for intrepid self-experimentation which led to many important discoveries about the human body and the nature of gases. He also experimented on his son, the equally famous J. B. S. Haldane, both for extending his father's interest in diving and as a key figure in population genetics and the development of the modern evolutionary synthesiseven when he was quite young. Haldane locked himself in sealed chambers breathing potentially lethal cocktails of gases while recording their effect on his mind and body.