Influence of tyramine (Ty) on behavioural changes in mice was studied and the following results obtained: 1) 30 min after Ty (160 mug, i.c.), brain noradrenaline and serotonin levels decreased while dopamine levels increased. 2) When Ty was injected i.c. into isocarboxazide (Iso) pretreated mice, spontaneous motor activity (SMA) measured by photo-cell counters method increased markedly but SMA by wheel cage method decreased. 3) When Ty was injected i.c. into Iso and p-chlorophenylalanine (p-CPA) (400 mg/kg, i.p., daily X 2) pretended mice, SMA measured by photo-cell counters method increased. While, SMA increased from 30 to 90 min after Ty, SMA measured by wheel cage method decreased till 30 min after Ty. 4) When Ty was injected i.c. into Iso and alpha-methyl-p-tyrosine (alpha-MPT) (125 mg/kg, i.p., daily X 2), SMA measured by photo-cell counters method decreased markedly when compared with Iso+saline treated group. When SMA was measured by wheel cage method, a difference between alpha-MPT+Iso-Ty treated group and Iso-saline group was not obtained. 5) When Ty was injected i.c. into p-CPA+alpha-MPT+Iso pretreated mice, SMA measured by photo-cell counters method did not show any increase compared with control group. 6) SMA produced by Iso-Ty in mice pretreated with haloperidol was significantly inhibited. A 50% dose of inhibition was seen with 0.3 mg/kg. From our results, it appears that an increase of SMA induced by Iso+Ty as revealed by the photo-cell counters method may be related to brain catecholamines and serotonin while a decrease of SMA in wheel cage method may be related to brain serotonin.