Undernutrition during early life causes deficits and distortions of brain structure. However, whether or not this includes a diminution of the total numbers of neurones remains uncertain. Recent advances in stereological techniques have made it possible to obtain unbiased estimates of total numbers of cells in well-defined biological structures. Rats were undernourished from conception to 90 postnatal days of age by standardised procedures. Groups of well-fed control and undernourished rats were anaesthetised and killed by intracardiac perfusion with fixatives at 30 and 90 days of age. Each olfactory bulb was serially sectioned at a nominal thickness of 100 microns on a vibratome. These sections were analysed by the Cavalieri principle to obtain estimates of the total volume of the olfactory bulb as well as the volume of its granule cell layer. The physical "disector" method was later used on serial 1-micron-thick toluidine-blue-stained sections to estimate the numerical density of granule cell neurones in the olfactory granule cell layer. These values were used to compute estimates of the total number of olfactory granule cell neurones for each animal. Thirty-day-old control and undernourished rats had between 2.6 and 3 million granule cell neurones in the olfactory bulb. By 90 days of age the number of granule cells had increased in both groups of animals to between about 4.2 and 5.2 million cells. Analysis of variance tests showed a significant main effect of age but not nutrition in these estimates. Although the interaction term did reach statistical significance, post hoc analysis did not reveal any differential effect of undernutrition between the two age groups examined.(ABSTRACT TRUNCATED AT 250 WORDS)