SNAKES seem to have evolved from burrowing lizards and are experiencing an impressive adaptive radiation. Terrestrial forms represent the probable ancestral type which gave rise to several lines of divergent radiation into aquatic and arboreal habitats1,2. These shifts in habitat may have affected blood pressure regulation in snakes which, because of their shape, are inordinately subject to the hydrostatic effects of gravity. By acquiring aquatic habits, some species have eliminated most of the hydrostatic stresses on the cardiovascular system; conversely, arboreal species are potentially exposed to serious problems of blood flow and perfusion pressure. Our study indicates that adaptive evolutionary changes in arterial blood pressure regulation occurred during the transitions from terrestrial to aquatic or arboreal life in snakes. There are distinct trends towards low arterial pressure, with reduced regulatory ability in aquatic species and towards high pressure, with good homoeostasis in an arboreal species.