The development of tolerance to the hyperactivating effects of methylamphetamine was studied in the laboratory rat. The results of Experiment I indicated that tolerance did develop. In Experiment II these indications were confirmed and the time course of tolerance was plotted. It appeared that the development of tolerance could be divided into two phases: an initial period of about one week during which activity increased, possibly due to accumulation of the drug or a metabolite, and a later period lasting about six weeks during which activity gradually fell to the first day of drug administration.