Several studies show that unilateral increase or decrease of the activity in the nigrostriatal dopamine system will cause an asymmetry in the movements and posture of animals. The symptoms may range from unilateral hypokinesia to vigorous rotation towards one side. In the present paper the nigrostriatal dopamine system of rats is unilaterally lesioned by intracerebral injection of 6-hydroxy-dopamine which produces an almost complete and selective lesion. The remaining contralateral dopamine system is activated by systemic treatment with amphetamine. The evoked rotational behavior is automatically registered as turns per minuteversus time in a specially designed ‘rotometer’ where the animal moves on a spherically shaped surface while connected to the registering device by a thin wire. The rotational behavior is highly reproducible and dose dependent, and it seems very probable that this rotational behavior is quantitatively related to the degree of dopamine receptor stimulation. The method is suggested for studies of different functional states in the nigrostriatal dopamine system.