1. A comparative study has been made of a population genetically homogeneous living in two adjacent but strongly different ecosystems.
2. Individuals were ‘marked’ with common serologic major genes.
3. The predominant strength of the environmental factors was demonstrated by the observation that when maintaining the genetic factor constant, hypertension appeared in the more advanced (‘acculturated’) Lowland populations.
4. The absence of hypertension was confirmed in Highland isolated primitive peoples.
5. Blood pressure correlation between relatives varied significantly with changing ecosystems.
6. The predominant strength of heredity was nevertheless demonstrated by the fact that in acculturated populations the genetic component of variance was found to be greater than the environmental one, and the dominant component accounted for nearly half the genetic variance.
7. Hypertension appears in the Lowlands to be exclusively associated with some specific genotypes of the Rhesus and MN systems.
8. The determination of blood pressure may be controlled by few major genes operating in given ecosystems under the permissiveness of environmental agents.