Several primary and secondary reactions were observed in rat hippocampus at various times following low doses of intraventricular kainic acid. The well documented fulminating lesion of subfield CA3a was evident, however, with the parameters of the present study, the acute neuronal death was restricted to areas which have been shown by others to have an exceptionally dense mossy fiber input. Neurons in subfields CA3c and CA4 also die, however, this was apparent only after a survival time of 2 weeks; prior to this, these cells went through a period of chromatolysis and dendritic swelling. In remaining subfields, principally 'CA2' and CA1, a third type of change was observed at longer intervals of 1 month. This was characterized by neuronal shrinkage and chromophilia and eventual cell loss. The present study provides additional observations on the actions of kainic acid in the hippocampus and may be relevant to the dynamic pathology of temporal lobe epilepsy.