Male rats of the Wistar strain were repeatedly given methamphetamine (MAP) at 0.5 mg/kg, s.c., 10 times at a fixed interval of 4 days in different environmental situations. The effects of MAP administration on ambulatory activity and neurochemical parameters in the brain were investigated. MAP (0.5 mg/kg) markedly increased ambulatory activity. The ambulation-increasing effect of MAP was progressively enhanced without accompanying stereotyped behaviors when the drug was repeatedly given in the activity cage. The once enhanced effect was considered to be irreversible. In addition, the pretreatment with MAP in the activity cage produced a decrease in the maximum density of 3H-spiperone binding sites in the striatum and that of 3H-WB4101 binding sites in the cortex and hippocampus, with decreases in catecholamine and increases in its metabolites. However, the enhancement of the ambulation-increasing effect of MAP and changes in the binding sites or catecholamine metabolism were scarcely observed when the drug was repeatedly given to the rats in the narrow cage where horizontal ambulation of the animals was strongly impaired. These results suggest that the enhancing effect of MAP is closely related to the environmental situations to which the animals have been exposed to under the drug effect. Moreover, it is indicated that there is a correlation between the enhancement of the ambulation-increasing effect of MAP and changes in the neurochemical parameters. The enhancing effect of MAP is supposed to have been partially caused by changes in cerebral catecholaminergic systems, in particular an increase in catecholamine turnover.