The threshold of carotid artery back pressure for the development of neuronal injury in the hippocampus was determined in gerbils following bilateral carotid artery occlusion of 5 or 10 min. Arterial back pressure was measured during ischemia at the left carotid bifurcation distal to the vascular occlusion, and neuronal injury evaluated one week after ischemia by counting the number of surviving neurons in the left hippocampal CA1 sector. With an arterial back pressure below 5 mm Hg, the mean density of surviving neurons decreased from 199 +/- 16/mm (mean +/- SD) to less than 21/mm both after 5 and 10 min ischemia (P less than 0.05). With a back pressure of between 5 and 15 mm Hg, neuronal density was 117 +/- 77/mm (not significantly different from control) after 5 min, and 24 +/- 18/mm (P less than 0.05) after 10 min ischemia. Above 15 mm Hg neither 5 nor 10 min ischemia produced significant neuronal damage. Thus, at threshold arterial back pressure, induction of neuronal injury in the hippocampus depends on the duration of ischemia, indicating progressive impairment of microcirculation with longer periods of ischemia.