Aggressive behaviour provokes a very high increase in plasma renin in aggressive male mice. We demonstrate that this occurs in mice with high as well as low concentrations of renin in the submaxillary glands, although the highest values were seen in the former. Plasma renin and angiotensinogen concentrations, as well as plasma renin activity, were measured before and after aggression. The theoretical plasma renin activity (generation rate of angiotensin I) was calculated from kinetic parameters and the renin and angiotensinogen concentrations. The Km|(Michaelis constant) was 3 µmol/l and the kcat(turnover number) was 0.15/s for the reaction between pure submaxillary mouse renin and mouse angiotensinogen. The measured plasma renin activity was much lower than that calculated from both the kinetics and the fall in angiotensinogen concentration. The decrease in angiotensinogen in vivo was independent of the renin concentration. Intraperitoneally injected renin mimicked the aggression-provoked renin increase and resulted in better agreement between measured and calculated parameters. This indicates, that during aggression, the mouse can inhibit and control the renin-angiotensinogen reaction.