The temporal evolution of irreversible neuronal damage from pilocarpine-induced seizures was studied by light microscopy. Neuronal cell death was judged on a 0-3 scale by estimating the percentage of acidophilic neurons in each of 23 brain regions. In addition, in the dorsal dentate hilus (CA4), quantitative cell counts of normal and acidophilic neurons were also performed. A few dead neurons (grade 0.5 damage) appeared in ventral hippocampal CA1 and CA3 regions after 20-min status epilepticus (SE). Slight-to-mild damage (grades 0.5-1.5) occurred in 14 and 12 brain regions after 40-min and 1-h SE respectively, and slight-to-moderate damage (grades 0.5-2.0) was found in 15 regions after 3-h SE. Twenty-four h and 72 h after 3-h SE, there was slight-to-severe damage (grade 0.5-3.0) in 22 and 21 regions respectively. Three-h SE produced more severe damage to 7 brain regions compared to 1-h SE, and 16 regions had more pronounced neuronal injury 24 h after rather than 0-4 h after 3-h SE. Eight brain regions had less damage 72 h compared to 24 h after SE, probably because of progressive neuronal lysis and dropout, but in mediodorsal and lateroposterior thalamic nuclei damage worsened from 24 to 72 h after SE. Neuronal cell counting revealed 20% acidophilic neurons in dorsal dentate hilus after 40-min SE and no difference between the 1-h and 3-h seizure groups (31% vs. 43% acidophilic neurons respectively). Among the 3 groups of rats with 3-h SE and varying recovery periods, the 24-h and 72-h recovery groups had higher percentages of acidophilic neurons (65% and 54% respectively) than the 0-4-h group (43%). Finally, the hippocampal CA2 region and dentate granule cell layer and the caudate-putamen, considered resistant to seizure-induced cell injury, were all damaged from SE lasting 40 min or more.