delta 9-Tetrahydrocannabinol (THC), one of the active compounds of marihuana, is known to induce drug dependence and tolerance, and its action is weaker than those of other abused drugs in humans and animals. Acute effects of THC, "high", "irritable" and "cognitive deficits" are more important than the drug dependence and tolerance. For this reason, we examined characteristics of abnormal behavior such as catalepsy-like immobilization, aggressive behavior including irritable aggression and muricide, and spatial cognition impairment induced by acute and chronic treatments of THC in rats. The catalepsy-like immobilization is related to a decrease in catecholaminergic and serotonergic neurons in the nucleus accumbens and amygdaloid nucleus and thus serves as a useful model for amotivational syndrome, one of cannabis psychoses. In aggressive behavior, muricide was determined by the housing condition. Muricide was induced if the rat was placed under an isolated housing condition within the period of the effect of single injection of THC. The behavioral change resembles exacerbation and flashback in humans. Spatial cognition is impaired by the interaction between cannabinoid (CB1) and 5-HT2 receptor in the dorsal raphe-hippocampal serotonergic neurons. Thus the abnormal behavior induced by THC can be a useful model for investigating mental function in humans and new drugs for the treatment of mental disorders.