Two new strains of inbred rats have been developed. One, WKHA, exhibits hyperactivity, and the other, WKHT, exhibits hypertension. Both of these traits are expressed in the SHR. By crossing spontaneously hypertensive rats (SHRs) with Wistar-Kyoto (WKY) controls, followed by recombinant selected inbreeding, we succeeded in genetically separating the hyperactivity from the hypertension in two new strains. Longitudinal studies indicate a persistence of hypertension without hyperactivity in WKHTs, and hyperactivity without hypertension in WKHAs, over at least 1 year. Ventricular enlargement, another characteristic of SHRs, was observed in adult WKHTs after the onset of hypertension; however, ventricles were already enlarged in normotensive WKHAs at 6 wk. The emergent behavioral profile of WKHAs indicates that they retain the hyperactivity trait and hyperreactivity to stress, and not some of the other behaviors of SHRs, such as poor habituation. Studies in WKHTs suggest that they are an improvement over SHRs as a model of genetic hypertension as they lack some prominent behavioral abnormalities. Nevertheless, the four genetically related strains (WKHA, WKHT, SHR, and WKY), used together, are considered most appropriate for seeking correlations of biological differences with either hypertension or hyperactivity.