Several problems concerning the ambulatory activity were discussed from the viewpoint of drug assay using about 700 adult male mice of ddG strain. Each response pattern for 180 approximately 400 min obtained from the temporal changes of ambulatory activity after administration of methamphetamine (1 approximately 16 mg/kg s.c.), d-amphetamine (1.25 approximately 5 mg/kg s.c.) or morphine (5 approximately 20 mg/kg s.c.) was recorded and used for the data analysis. All the above mentioned drugs markedly accelerated the activity when the doses were optimal. However, the individual response to the drugs sometimes reached a 40- approximately 70-fold difference, even though both animal and measurement conditions were fixed as constant way as possible. The time when the experiments took place did not explain the individual differences, nor did the season or body weight of the animal. However, there was the possibility of colony differences. Animals with relatively high activity prior to drug administration tended to respond more forcibly after administration of the drugs. It was found from the statistical investigation that constant observation of over 1 hr using at least 15 animals was required in order to determine a standard effect of the drug. Furthermore, the accelerating effect of methamphetamine and d-amphetamine on the activity was observed to be dose-dependent when below 4 and 5 mg/kg was given, respectively. On the contrary, the effect decreased dose-dependently when higher doses were given as stereotyped behavior such as head-swaying, continuous sniffing and circular movements in opposition to the horizontal movements also developed dose-dependently. As the qualitatively different behavior could be simultaneously measured, the individual differences were more apparent.