Weanling male and female golden hamsters were caged in matched-weight pairs on removal from their mothers. There were pairs of littermates and non-littermates of each sex. One partner of each pair was dyed on the chest with a black commercial hair-dye. Over 20 days, dyed animals of both sexes had a significantly higher growth rate than their untreated partners. In observations carried out between days 10-20 of the test period, dyed animals were found to be aggressively dominant over their untreated partners, and showed more aggressive behaviour. It is suggested that the growth of untreated animals was inhibited as a "stress" response either to the presence of the supra-normal threat stimulus, or to the aggressive relationship which resulted from it.