This study investigated the behavior of hypertensive-prone and nonhypertensive-prone rat strains to see whether there are differences in behavioral reactivity to environmental stimulation. Of primary interest was general activity, because investigators have assumed it to be an index of reactivity to environmental stimulation and to be correlated with the elevation of blood pressure in spontaneously hypertensive rats (SHR) but not in normotensive Wistar-Kyoto (WKY) rats. Therefore, general activity was assessed in SHRs and WKYs as a function of walking and rearing in an open field (Experiments 1 and 2) and crossings in a shuttlebox test (Experiments 3, 4, and 5). Activity was assessed under a constant level of environmental stimulation in the open field (Experiment 1) and shuttlebox (Experiment 3) or under varying degrees of stimulation in the open field (Experiment 2) and shuttlebox (Experiments 4 and 5). In the open field and the shuttlebox, SHR activity was above WKY activity when the degree of environmental stimulation was constant. However, when stimulation was manipulated, the SHR activity level was similar for all intensity levels, while the WKY activity was inversely related to stimulus intensity. These results suggest that while the SHRs may generally be more active than WKYs, WKYs display a greater behavioral reactivity to environmental stimulation than SHRs.