Measures of extraversion and neuroticism in rats are related to aggressive behaviour in two strains of rats. Isolated rats and rats reared together, of the Sprague-Dawley and Wistar strains, were submitted to an aggressiveness test. The results show that isolation generates and/or increases mouse-killing behaviour only in Wistar rats which are more emotional (neurotic) than Sprague-Dawley rats. The two strains do not differ significantly in their open-field ambulation (extraversion), although Wistar rats tend to be less ambulatory. Within the Wistar strain, the results showed the same trend, that is, the more emotional and less ambulatory rats (dysthymics) are the ones who became killers after isolation. The Sprague-Dawley strain does not show any significant aggressive behaviour after isolation. © 1985.