A morphometric analysis of constituents of the rat hippocampus was performed in the different hippocampal regions that are known to be vulnerable in various degrees to experimental global ischemia. In the hippocampus formation of the normal Fisher rat the number and areas of neuronal nuclei and of their constituents, nucleoli and AgNOR particles, were counted and measured. Nuclear and nucleolar size were different in the various parts of the Ammon's formation. AgNORs, i.e. silver stainable proteins that represent actively rRNA transcribing genes, equally varied in number and extension in the different hippocampal subfields. In granule cells of the dentate gyrus and in pyramidal cells of CA 1, CA 3 and CA 4, AgNORs usually occupy half of the nucleolar area. Using the determined values an estimate of the length and volume of the neuronal layers within the different parts of the rats Ammon's horn was performed. After the development of delayed ischemia all measured structures were significantly smaller despite the fact that only nuclei with well preserved outlines could be analysed. They represent only 5-10% of controls. Nuclear sizes were reduced between 20 and 55%. The strongest reduction was observed in the transition zone between CA 1 and CA 3, the least in CA 3. Nucleoli and AgNORs equally were considerably smaller than in normal cells. For AgNORs a skewed distribution to the left resulted, whereas the nuclear size had normally distributed shape with peak at lower values. We conclude that an architecture of both nucleoli and AgNORs exists in the hippocampal subfields in addition to the known cytological architecture. Under ischemic conditions, even the healthy looking cells remain damaged to a variable extent in the different subfields corresponding to their selective vulnerability. There are some indications that damage to nucleoli and AgNORs is earlier and stronger than to nuclei. A time course of AgNOR damage can be constructed.