This article presents complementary data on the relationship between the intensity of offensive and defensive aggression that was investigated by means of the computerised Pulkkinen Aggression Machine (PAM) paradigm [Juuja¨rvi et al., 2001; Aggr. Behav. 27:430-445]. The recording of the electrocardiogram was conducted in a sample of 109 children (61 boys and 48 girls) while they completed the PAM. Across the conditions of controlled aggression, the simulated attack and defence evoked a mean increase of HR by 2.7 beats per minute (bpm), but the variation between children was substantial (À10.9 to+11.4 bpm). Children who showed a strong HR increase, moderate HR increase, or HR decrease were compared on heart rate variability (HRV) and intensity of (defensive) reactive aggression. Variation of mean HR reactivity was generally related to changes in parasympathetic activity since the power of HRV changed in the high (0.15-0.40 Hz) rather than the low (0.04-0.15 Hz) frequency band. The correspondence between the intensity of offensive and defensive aggression was weakest among those children who displayed either a strong increase or a decrease of HR as compared to those children who exhibited only moderate increase of mean HR. The stronger the children displayed an increase of HR during the PAM, the lower they were rated on the teacher rating scales for depressive symptoms, social anxiety, and aggression. Together, these results suggest that the relationship between the intensity of physical provocations and reactive aggression was moderated by dispositional negative emotionality and its regulation. Aggr. Behav. 32: 99-109, 2006. r