Spontaneously hypertensive rate (SHR) generally exhibit higher performance in avoidance tasks than Wistar-Kyoto control rats (WKY) do. The SHR are more active than the WKY after transfer to a novel environment and after electric footshocks. The present study examined the performance, behavior, and plasma corticosterone of young male SHR and WKY in a passive-avoidance task in which high levels of locomotor activity do not facilitate performance. The effects of three levels of discriminability of the safe area were studied. Passive-avoidance performance was statistically significantly lower in the SHR, but the difference was small. There was no difference between the SHR and the WKY in post-test plasma-corticosterone levels, and there was no relationship between ambulation and plasma-corticosterone levels. The behavioral differences between the SHR and the WKY varied as a function of the discriminability of the safe area. Specifically, in the low-discriminability condition, ambulation, but not rearings, was equally suppressed in the SHR as in the WKY and there was no difference in immobility. In the high-discriminability condition the SHR exhibited more ambulation, more rearing, and less immobility than the WKY. These data confirm that the SHR exhibit an increased tendency for facilitation of locomotor activity during activation compared to the WKY. Because the passive-avoidance performance of the SHR was lower than that of the WKY, these data suggest that previous reports of superior avoidance learning in the SHR reflect their elevated level of locomotor activity rather than an improved ability to acquire conditioned responses. Ability to discriminate stimuli seems to be normal in the SHR since ambulation was modified according to stimulus parameters.