The aim of this study was to compare the effects of malnutrition on behavioral development in mice with those due to heredity obtained in a previous study. To this end behavioral development was assessed using a developmental time scale which allowed comparison of the two studies. Pregnant B6D2F1/J mice were maintained on either a protein restricted or normal protein diet from Day 7 to Day 24 postconception, at which time the low protein group was returned to the normal diet until behavioral testing at Day 32. Behavioral development in the control group replicated the values obtained in previous studies, whereas the malnourished pups were retarded by 1.4 days. This was of the same order of magnitude as that reported previously for maternal inbreeding. Measures of maternal and pup body weight at birth, Day 24 and Day 32 indicated that although the malnourished animals were initially significantly lighter in weight they had recovered by Day 32. Litter size however, declined between birth and time of behavioral testing in the malnourished group.