The stroke-prone spontaneously hypertensive rat (SHRSP) is vulnerable to delayed neuronal death (DND) in the CA1 subfield of the hippocampus after the transient forebrain ischemia by the occlusion of the bilateral carotid arteries. The present study was designed to show that the genetic factors independent of high blood pressure contributed to the high incidence of DND in SHRSP. Male rats of the four strains, SHRSP/Izm, SHRSP/Ngsk, SHR/Izm and a congenic strain for the blood pressure quantitative trait locus on chromosome 1 [SHRSP.WKY-(D1Wox29-D1Arb21)/Izm]were used in the experiments. At 13 weeks of age, the bilateral carotid arteries of rats were occluded for 10 min under anesthesia with their body temperature kept at 37 degreeC. Seven days after the transient ischemia, the loss of the pyramidal cells in the CA1 was evaluated histologically. In some experiments, the blood flow was monitored with a laser Doppler flowmeter during the transient ischemia. The blood pressure in SHRSP/Izm was significantly greater than that in the other three strains. The incidence of DND, however, was not significantly different among SHRSP/Izm, SHRSP/Ngsk and the congenic strain (82, 74 and 65%, respectively), while SHR/Izm showed a significantly lower incidence (20 percent). Neither a significant correlation between the incidence of DND and the blood flow reduction during the occlusion, nor a significant inter-strain difference in the blood flow reduction was observed. The genetic factors independent of high blood pressure may contribute to the greater susceptibility to DND in SHRSP.