We have previously shown that the numerical density of dentate granule and CA3 pyramidal cells of adult rats is reduced after lengthy periods of low-protein diet. In this study, the total number of these neurons was estimated, together with those for the hilar and CA1 pyramidal cells in order to obtain a complete and unbiased insight into the effects of malnutrition and rehabilitation from malnutrition on the structure of the hippocampal formation. Groups of 2-month-old rats were fed a low protein diet (8% casein) for 6, 12 and 18 months and compared with age-matched control and recovery rats. The recovery group was fed a low protein diet for 6 months and then switched to normal diet during the same period. Total numbers of neurons of each hippocampal region were calculated from their numerical density, estimated with the physical disector, and from the volume of the respective cell layers, after correction for the tissue shrinkage factor. The total number of granule, hilar, CA1 and CA3 pyramidal cells was reduced in all groups of malnourished rats including the recovery group. No differences were found between malnourished and recovery groups. These findings indicate that a prolonged low protein diet, started in adult life, leads to a deficit in neuronal numbers in the hippocampal formation, and that it may also disrupt the normal process of cell acquisition in the dentate gyrus. Moreover, our data support the view that the morphological alterations induced by a low protein intake are irreversible.