A longitudinal study of rural-urban migration from a low blood pressure community was completed in 1985. It is suggested that the rapid increase in blood pressure observed with acculturation in this study constitutes a new dynamic model for the early phase of human essential hypertension. Analysis of data derived from this study indicates that increases in dietary sodium, weight gain and sympathetic tone (possibly reflecting acute environmental stress) are important determinants of the early blood pressure rise. It is proposed that a defence reaction occurs in response to environmental stress which maintains or increases cardiorenal sympathetic outflow, thereby overriding the negative feedback inhibition of sympathetic activity which otherwise occurs when dietary sodium is suddenly increased. The antinatriuretic response results in an early volume-dependent rise in blood pressure. We also propose that variation in individual blood pressure responses to migration may be explained not only by quantum differences in the two environmental factors, but also by a genetically-determined failure of adaptation to the combined effects of these stimuli on renal tubular reabsorption of sodium.