Mice selected genetically for high and low blood pressure (BP) were compared with regard to heart weight and heart/body ratios. Two experiments were performed with mice ranging in age from 1.3 to 9 months and 11 to 23 months respectively. In a third experiment C57BL/6J mice were compared to the high and low BP mice. Heart/body ratios and heart weights, adjusted fo body weight via covariance analysis, were significantly greater for the high BP mice, but no Age x BP Genotype interaction was observed. Results were discussed in terms of a possible relationship between heart weight and BP via genetic linkage or pleiotropy. The possibility was also raised that compensatory mechanisms for high blood pressure, e.g., cardiac hypertrophy, may operate very early in development for animals who are hypertensive by virtue of selective breeding for blood pressure extremes.