A colony of 26 chimpanzees given a fruit and vegetable diet of very low Na and high K intake were maintained in long-standing, socially stable small groups for three years. Half of them had salt added progressively to their diet during 20 months. This addition of salt within the human dietetic range caused a highly significant rise in systolic, mean and diastolic blood pressure. The change reversed completely by six months after cessation of salt. The effect of salt differed between chimpanzees, some having a large blood pressure rise and others small or no rise. These results in the species phylogenetically closest to humans bear directly on causation of human hypertension, particularly in relation to migration of preliterate people, with low Na diet, to a Western urban lifestyle with increased salt intake. The hedonic liking for salt and avid ingestion was apt during human prehistory involving hunter-gatherer-scavenger existence in the interior of continents with a scarcity of salt, but is maladaptive in urban technological life with salt cheap and freely available.