Previous studies were concerned with distribution curves of values of brain parameters (weight, DNA or cell number, and protein) in a population of 720 neonatal rats. These values followed normal distribution curves, and the tails of such curves, with values in excess of two standard deviations (SD) above (right hand) or below (left hand) the mean (X), were statistically significant. In the present work a larger sample (1,948) of neonatal rats, with particular reference to those whose brain parameters exceeded three SD above or below X, respectively, was examined. Such occurrences are rare; they are somewhat skewed with respect to numbers expected in the tails of a normal distribution curve. For DNA the highest deviations from X were for 'maximal' brain, +3.62 SD (right tail); and for 'minimal' brain, -4.33 SD (left tail). Further study suggested that one of the factors increasing the maximal values might be a lower number in the litter, and one of the factors decreasing the minimal values was a lower maternal weight at conception. Thus the factors affecting maximal and minimal brains are not the same. The occurrence of such individuals with maximal or minimal values of neonatal brain parameters is of particular interest if these parameters are correlated with behavioural performance.