Blood pressure and body weight rise with age, and hypertension is hypothesized to result from an early disorder of growth. We used the statistical technique of spline fitting to examine patterns of growth and rising blood pressure in individual inbred rats of the Wistar-Kyoto, spontaneously hypertensive (SHR), and stroke-prone SHR strains. Our results indicate that growth is phasic and that a well-defined growth spurt peaks at approximately 50 days of age in each strain. In addition, the time of maximal weight gain corresponds to the onset of a blood pressure spurt. The timing of this blood pressure rise was similar in the three strains, but the maximal rate of blood pressure rise was significantly higher in the hypertensive strains. Our results suggest that growth and rising blood pressure are intimately linked. Blood pressure appears to rise in response to a growth spurt, suggesting that there are growth-related events that trigger the cardiovascular response.