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1. Aggressive behaviour was elicited in rats that had been deprived of food for 20 h daily (starved), by chronic administration of Cannabis sativa extract or (-)-Delta(9)-trans-tetrahydrocannabinol.2. The influence of intraperitoneal (i.p.) or oral glucose administration, cold environment, acidosis, and corn, and protein-free diets on this aggressiveness was studied.3. Intraperitoneal injections of glucose (100-1,600 mg/kg) did not alter the aggressiveness induced by marihuana in starved rats; glucose given orally, however, blocked this behaviour.4. Low temperature (14 degrees C) strongly potentiated the aggressive behaviour induced by marihuana in the starved rats.5. Lactic acid in doses capable of potentiating thiopental anaesthesia, failed to alter the marihuana-aggressiveness of starved rats or to facilitate this effect of marihuana in rats fed ad libitum. The same negative results were obtained with ammonium chloride.6. In rats fed ad libitum with protein-free or corn diets, marihuana administered chronically did not elicit aggressive behaviour. However, aggressiveness appeared when rats were fed for only 2 h daily on those diets.7. The results suggest that the stress of hunger (and not hypoglycaemia, acidosis or lack of specific nutrients due to starvation) is the factor that facilitates the development of aggressive behaviour by chronic administration of marihuana.