The present study is concerned with the demonstration of the relationship between the behavior and the brain concentration of noradrenaline resulted from pretreatment of amphetamine in isolated or aggregated rats. The experimental subjects were rats weighing from 120g to 200g housed 1, 2, and 6 in a cage. Analeptic activity of amphetamine was measured by determining the sleeping time induced by pentobarbital sod. The noradrenaline content in brain was determined with Aminco-Bowmann s spectro-photofluorometer by Lee s modification of Shore and Olin method. Results: 1) The analeptic activity of amphetamine on the sleeping time induced by pentobarbital sod. was more increased in the grouped rats than in isolated animal. 2) In being isolated and grouped rats, the sleeping time induced pentobarital sod, was markedly prolonged by pretreatment of amphetamine. 3) Means of housing rats, e.g., isolation or aggregation did not seem to affect the brain noradrenaline depleting action. 4) Repeated daily parenteral administration of amphetamine sulfate for a period 1 to 3 weeks resulted in decrement of brain noradrenaline concentration in being isolated and grouped rats. 5) The prolongation of sleeping time of the isolated or aggregated rats, when pretreated with amphetamine, compared with that of stock rats, seems to be attributable rather to the means of housing than the variation of the noradrenaline caused by amphetamine.