In 1969 outbred Sprague-Dawley rats were selected for high systolic blood pressure using an indirect plethysmographic technique in pre-warmed unrestrained conscious rats. Three pairs were originally selected, and selection was continued with brother x sister mating. Strain LN was maintained as a normotensive control, and LL as a hypotensive strain. (see Vincent et al 1984, and Vincent and Sassard, 1994 for reviews). This LH is the parent to LH substrains maintained in other institutions.