Muricide behaviour (MB) in rats may be induced either by Mg deficiency (1) or by intraperitoneal injection of high dosage (11 mg/kg b.w.) of Δ9-Tetrahydrocannabinol (THC) (2). The aim of this work is to study THC influence at infraeffective dosages upon MB in Mg-deficient rats. OFA male rats, 5 per cage, were fed a Mg-deficient 50 ppm diet during 40 days. Treated rats (10/group) were injected intraperitoneally with 2, 4 and 8 mg/kg THC (1% aqueous solution in Tween 80), control ones with vehicle, then kept 1 per cage. MB of each rat (350 +/- 25 g) was observed 1 hour later against a OF1 male 30 g mouse by determination (in s) of the attack latency and of the duration of the attacks on the living then on the dead mouse.The attack latency (control Mg-deficient: 14.7 +/- 0.6 s) decreased significantly with THC doses (8 mg/kg: 3.4 +/- 0.3 s), while the duration of the attack on the living mouse (control: 55.3 +/- 1.2 s) increased dramatically from 4 mg/kg (100.6 +/- 12.8 s). The duration of the attack on the dead mouse (control: 69.3 +/- 1.0 s) was increased at 2 mg/kg THC (100.2 +/- 12.9 s), then decreased for the 2 higher doses (40.6 +/- 5.7 and 18.6 +/- 3.5 s respectively).In conclusion on MB, aggressiveness is enhanced by THC low doses in magnesium-deficient rats. So the association of low doses of THC and Mg deficiency in rats may present muricide behaviour as an in vivo sensitive model for central THC antagonists studies.